Which of the following will have the most negative and the least negative electron gain enthalpy?

Halogens have the highest negative value of electron gain enthalpy. Here Cl and F both are halogens but electron gain enthalpy of Cl is more negative than F because of the very small size of F which causes interelectronic repulsion among its electrons. 
Cl has the most negative one. P has the least negative one due to the larger size.