Answer:- C. Halogen
Explanations:- Metals loses electrons to make cations where as non metals gain electrons to make anions.
So, Alkali metals and Transition metals are eliminated as they lose electrons to make cations.
Boron is a non metal but rest of the elements of its group are post transition metals and they also lose electrons to make cations.
Halogens are non metals are highly electron negative in nature so they gain electrons to form anions.
Hence, the right choice is C. Halogens.