When the first electron gain enthalpy of oxygen is –141 kJ/mol, its second electron gain enthalpy is

We know that When an electron is added to a gaseous atom, energy is released to form a negative ion. Thus, the first electron gain enthalpy, a change in enthalpy, is negative.

However, when an electron is introduced, the mono negative ion faces repulsion from the electron, therefore energy must be supplied to overcome the repulsion. As a result, the second electron gain enthalpy is positive since the second electron requires energy to be added.

Although an oxygen atom having six valence electrons requires two electrons to become stable, the formation of $O^{-}$anion by a gain of an electron offers repulsion to the second electron in order to make the second electron gain enthalpy positive, energy must be supplied.
Hence, option(C) is the answer.