**Answer:**

The electron gain enthalpy is the energy released when an atom gains an electron to form a negative ion. It is a measure of the tendency of an atom to accept an electron.

To determine which element will have the most negative electron gain enthalpy and which one will have the least negative, we need to consider the factors that influence electron gain enthalpy.

1. **Nuclear charge**: The greater the nuclear charge, the stronger the attraction for the incoming electron, resulting in a more negative electron gain enthalpy.
2. **Atomic size**: The smaller the atomic size, the closer the valence electrons are to the nucleus, resulting in a stronger attraction for the incoming electron and a more negative electron gain enthalpy.

Considering these factors, we can analyze the elements given in the options:

a) Cl, P: Chlorine (Cl) has a higher nuclear charge than Phosphorus (P), so it will have a greater attraction for the incoming electron. Additionally, chlorine is smaller in size compared to phosphorus. Therefore, chlorine will have the most negative electron gain enthalpy, and phosphorus will have a less negative electron gain enthalpy.

b) P, Cl: As mentioned above, chlorine (Cl) has a higher nuclear charge and is smaller in size compared to phosphorus (P). Therefore, chlorine will have a more negative electron gain enthalpy, and phosphorus will have a less negative electron gain enthalpy.

c) P, S: Sulfur (S) has a higher nuclear charge and is smaller in size compared to phosphorus (P). Therefore, sulfur will have a more negative electron gain enthalpy, and phosphorus will have a less negative electron gain enthalpy.

d) None: The answer cannot be "none" since we have already determined that chlorine will have the most negative electron gain enthalpy.

In conclusion, the correct answer is option 'A': Cl will have the most negative electron gain enthalpy, and P will have the least negative electron gain enthalpy.

P is having more metallic nature among all so it is least electronegative.among chlorine and fluorine since fluorine is a smaller element so there are inter electronic repulsions between the core electrons and extra electron added. so chlorine has most negative electron gain enthalpy