The correct answer is A. P.
Electron gain enthalpy is the energy released when an electron is added to a neutral atom in its ground state to form a
negative ion. The electron gain enthalpy of an element is a measure of its electronegativity. The more electronegative an element is, the more negative its electron gain enthalpy will be.The elements in the periodic table are arranged in order of increasing atomic number. As you move from left to right across a period, the atomic number increases and the electronegativity increases. This is because the elements on the right side of the periodic table have more protons in their nucleus than the elements on the left side. The more protons in the nucleus, the stronger the attraction between the nucleus and the electrons in the valence shell. This makes it more difficult to add an electron to the atom, and therefore the electron gain enthalpy is more negative.
The elements in a group have the same number of valence electrons. As you move down a group, the atomic radius increases and the electronegativity decreases. This is because the electrons in the valence shell are further away from the nucleus, and therefore the attraction between the nucleus and the electrons is weaker. This makes it easier to add an electron to the atom, and therefore the electron gain enthalpy is less negative.
P, S, Cl, and F are all in the same group (group 16). P has the least negative electron gain enthalpy because it has the largest atomic radius. The larger the atomic radius, the further away the electrons in the valence shell are from the nucleus. This makes it easier to add an electron to the atom, and therefore the electron gain enthalpy is less negative.