Explanation:

The correct answer is option 'C', which states that the vapour absorption refrigeration system works using the affinity of a substance for another substance.

Vapour Absorption Refrigeration System:
The vapour absorption refrigeration system is an alternative to the more commonly used vapour compression refrigeration system. It utilizes the principle of absorption, where a refrigerant vapor is absorbed into a liquid, and then the refrigerant is released by heating the liquid to produce a refrigeration effect.

Affinity of a Substance:
Affinity refers to the attraction or chemical bonding between two substances. In the case of a vapour absorption refrigeration system, the affinity between the refrigerant vapor and the absorbent liquid is crucial for the system to function.

Working of Vapour Absorption Refrigeration System:
The working of a vapour absorption refrigeration system involves several steps:

1. Evaporation: The refrigerant (ammonia, in most cases) evaporates at low pressure and temperature, absorbing heat from the surroundings.

2. Absorption: The evaporated refrigerant vapor is then absorbed by the absorbent liquid (usually water), forming a strong chemical bond between the two substances. This process occurs at a relatively low temperature and pressure.

3. Desorption: The absorbent-refrigerant mixture is then heated to release the refrigerant vapor from the absorbent. This is achieved by applying heat using a heat source like a gas burner or waste heat.

4. Condensation: The released refrigerant vapor is then condensed back into a liquid by rejecting heat to the surroundings. This occurs at a higher pressure and temperature.

5. Expansion: The high-pressure liquid refrigerant is then expanded through a throttle valve or an expansion device, which reduces its pressure and temperature.

6. Evaporation: The expanded refrigerant liquid now enters the evaporator, where it evaporates again by absorbing heat from the refrigerated space or load.

7. Repeat: The cycle is repeated continuously to maintain the desired cooling effect.

Affinity in Vapour Absorption Refrigeration System:
The affinity between the refrigerant vapor and the absorbent liquid is what drives the absorption and desorption processes in the system. The absorbent has a strong attraction or affinity for the refrigerant vapor, allowing it to absorb and release the refrigerant efficiently. This affinity ensures that the absorbent can effectively capture the refrigerant during absorption and release it during desorption, enabling the refrigeration cycle to operate.

In conclusion, the correct answer is option 'C' because the vapour absorption refrigeration system relies on the affinity of a substance (absorbent) for another substance (refrigerant) for its functioning.

The Correct Answer is option 'C': Affinity of a substance for another substance.

Explanation:
A vapour absorption refrigeration system is a type of refrigeration system that is based on the principle of absorption of a refrigerant by a suitable absorbent. It involves the use of two substances, a refrigerant and an absorbent, which have a high affinity for each other.

Affinity of a substance for another substance:
The term "affinity" refers to the chemical attraction or bonding between two substances. In the context of a vapour absorption refrigeration system, it refers to the strong attraction between the refrigerant and the absorbent.

Working Principle of Vapour Absorption Refrigeration System:
The working principle of a vapour absorption refrigeration system involves the following steps:

1. Evaporator:
- The refrigerant, which is in the form of a low-pressure vapour, enters the evaporator.
- The evaporator is a heat exchanger where the refrigerant absorbs heat from the surroundings and evaporates, thus cooling the surroundings.

2. Absorber:
- The refrigerant vapour leaving the evaporator enters the absorber, where it comes into contact with the absorbent.
- The absorbent has a high affinity for the refrigerant and readily absorbs it, forming a strong solution.

3. Generator:
- The strong solution from the absorber is pumped into the generator.
- In the generator, the solution is heated, usually using a heat source such as a gas burner or waste heat, causing the refrigerant to separate from the absorbent.

4. Condenser:
- The refrigerant vapour leaving the generator enters the condenser, where it is cooled and condensed into a high-pressure liquid.

5. Expansion Valve:
- The high-pressure liquid refrigerant passes through an expansion valve, which reduces its pressure and temperature.

6. Evaporator:
- The low-pressure liquid refrigerant then enters the evaporator again, and the cycle repeats.

Key Points:
- The affinity between the refrigerant and the absorbent is crucial to the operation of a vapour absorption refrigeration system.
- The absorbent must have a high affinity for the refrigerant so that it can readily absorb it and form a strong solution.
- The strong solution in the absorber is further separated in the generator, allowing the refrigerant to be condensed and reused in the system.
- The absorption process in a vapour absorption refrigeration system is driven by the affinity between the refrigerant and the absorbent.
- This system is commonly used in large-scale refrigeration applications, where waste heat or other low-grade heat sources can be utilized as the energy input for the generator.