The correct order of the electron affinity for one-electron gain of the elements is

​

Correct option is C

​The electron affinity (E_ea​) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion.

Atoms whose anions are more stable than neutral atoms have a greater E_ea. Chlorine most strongly attracts extra electrons; neon most weakly attracts an extra electron. 

E_ea generally increases across a period (row) in the periodic table prior to reaching group 18. This is caused by the filling of the valence shell of the atom; a group 17 atom releases more energy than a group 1 atom on gaining an electron because it obtains a filled valence shell and therefore is more stable. In group 18, the valence shell is full, meaning that added electrons are unstable, tending to be ejected very quickly.

Counterintuitively, E_ea does not decrease when progressing down most columns of the periodic table.

Option a belongs to group 17

Option b belongs to group 15

Option c belongs to group 16

Option d belongs to group 1

​