Answer:
F, Fluorine
Step-by-step explanation:
Na^+ and Mg^2+ have both lost enough electrons to bring their electron shells to that of Neon, Ne. (1s^2, 2s^2, 2p^6)
O^2- gained 2 electrons to also conform to neon's electron configuration.
F still retains its 1s^2, 2s^2, 2p^5 configuration, which would result in a slightly smaller radius as the 2p^5 electrons can pack slightly closer to the nucleus before being repelled by the negative charge of their neighboring electrons.