a. If XF is covalent, X couldn't be a metal. Because covalent bonding occurs between nonmetals. So this alternative is False.
b. If XF is ionic, X could be a nonmetal. Also False. Ionic bonding occurs between a metal and a nonmetal. In this case, we already have a nonmetal, which is F.
c. If XF is covalent, the name uses prefixes. True. When we have a covalent compound, the name of the nonmetal on the left has a prefix indicating the number of atoms of it.
d. If XF is covalent, X has a charge of +1. False. This would happen if it was ionic.
Answer: Alternative "C"