A. X is an element of group 5A. False. X is bonded to Nitrogen, which is in group 5A. We have 3 atoms of X, so it needs just one electron to complete the octet or it is hydrogen. If it needs just one electron, it must be an element of group 7A.
B. The number 3 in NX3 and AlX3 is the charge of X. False. It is the number of atoms of X in the molecule or compound.
C. X could be a halogen. True. As we see it, X needs just one electron to complete the octet, so it can be a halogen (group 7A).
D. The number 3 in NX3 and AlX3 is the group number of X. False. As we said before, it is the number of atom of X in the compound.
Answer: Alternative "C"