The electron affinity of an atom typically depends on the
a. mass of the atom's nucleus.
b. number of electrons already present in the atom.
c. number of neutrons in the nucleus of the atom.
d. availability of an unfilled orbital in the atom.

Option D: availability of an unfilled orbital in the atom.

This is true because the atoms will try to complete the octete (8 electrons in the last p level)

The activity series is shown in the attached picture. if the element is below the other, then you can displace the other. following the activity series as a guide, the reactions are as follows: nickel and aluminum chloride3 ni + 2 alcl₃  → 3 nicl₂ + 2 albarium and copper bromideba + cubr  →  no reaction because cu is less oxidizable than bapotassium nitrate and aluminum3 kno₃ + al  → al(no₃)₃ + 3 ksodium phosphate and zincnaso₄ + zn  → znso₄ + na