The increase in size of an anion is due to the increase in repulsion of:

Explanation

Option A: Electron-proton is not the correct answer as the number of protons in the nucleus remains the same when an atom gains an electron to become an anion.Option B: In general, anions are larger than the corresponding neutral atom, since adding electrons increases the number of electron-electron repulsion interactions that take place. Cations are smaller than the corresponding neutral atoms, since the valence electrons, which are furthest away from the nucleus, are lost.Option C: Electron-nucleus is also not the correct answer as the attraction between the negatively charged electrons and positively charged nucleus increases when an atom gains an electron to become an anion.Option D: Proton-nucleus is not the correct answer as the number of protons in the nucleus does not change when an atom gains an electron to become an anion.