Which of the following has the highest electron affinity? (AFMC 2011)

Electron gain enthalpy or electron affinity is the amount of energy released when an electron is added in a neutral gaseous atom. The trend of electron affinity decreases down the group due to the increase in atomic size and it increases from left to right in the period due to the decrease in atomic size. As fluorine ion Fand oxygen ion Ohas already accommodated an electron in it. The addition of a second electron is opposed by coulumbic repulsion. And the electron gain enthalpy of sodium is less than oxygen (O). Therefore, Oxygen (O) has the highest electron affinity.