In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. It is that temperature and pressure at which the sublimation curve, fusion curve and the vaporisation curve meet.

Answer is triple point because at this point all three states ( solid,liquid and gas ) Co - exist.

Triple Point: Coexistence of all three phases

The correct answer is option 'B': Triple point.

The triple point is defined as the temperature and pressure at which all three phases of a substance coexist in equilibrium. It is a unique point on the phase diagram of a substance where the solid, liquid, and gas phases can coexist in perfect balance.

Here is a detailed explanation of why the triple point is the correct answer:

1. Definition of the Triple Point:
- The triple point is a specific set of conditions (temperature and pressure) at which all three phases of a substance can exist simultaneously.
- At the triple point, the solid phase, liquid phase, and gas phase are in equilibrium, meaning they coexist in a specific ratio.

2. Phases of a Substance:
- A substance can exist in different phases, such as solid, liquid, and gas, depending on the temperature and pressure.
- At low temperatures and high pressures, substances tend to be in the solid phase, where particles are tightly packed and have little freedom of movement.
- At high temperatures and low pressures, substances tend to be in the gas phase, where particles are far apart and have high mobility.
- In between these extreme conditions, substances can exist in the liquid phase, where particles are close together but still have some freedom of movement.

3. Phase Diagram:
- A phase diagram is a graphical representation of the different phases of a substance as a function of temperature and pressure.
- The phase diagram typically shows the solid, liquid, and gas regions, as well as the boundaries between them.
- The triple point is represented as a specific point on the phase diagram, indicating the conditions at which all three phases can coexist.

4. Importance of the Triple Point:
- The triple point is crucial for defining temperature scales, such as the Kelvin scale.
- The Kelvin scale is defined by setting the triple point of water as exactly 273.16 Kelvin (0.01 degrees Celsius).
- By using the triple point of water as a reference, scientists can accurately calibrate and measure temperatures.

In conclusion, the temperature and pressure at which all three phases of a substance coexist is called the triple point. It is a unique point on the phase diagram where solid, liquid, and gas phases are in equilibrium. The triple point is important for defining temperature scales and serves as a reference for accurate temperature measurements.