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

$Cl$ has the most negative electron gain enthalpy and $P$ is the least negative.

The biggest negative value of electron gain enthalpy is seen in halogens. Here, both Cl and F are halogens, but Cl's electron gain enthalpy is more negative than F's due to F's extremely small size, which results in interelectronic repulsion between its electrons.
Cl has the worst one. Due to its higher size, P has the least negative one.

Hence, option D is correct.