The process requiring absorption of energy is:
(1) F ——>  F
(2) H —–> H
(3) CI —–> Cl
(4)  O —–> O2-

Answer: (4)
When oxygen atom (O) gains one electron, it becomes a negatively charged oxygen ion (O). This process does not require much energy because the atom is still relatively neutral and the electron can easily be added.
But, when we try to add a second electron to the already negatively charged oxygen ion (O), then we have a problem. The two negatively charged electrons repel each other, like two magnets with the same pole. To overcome this repulsion and bring the second electron closer to the oxygen ion, we need to add energy. So, the process of adding the second electron to the oxygen ion requires energy because we have to work against the repulsion between the negatively charged particles. This energy absorption makes the overall process endothermic.

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