While Diamond structure will present Carbon bonded to four other Carbon atoms, Graphite structure will not, as it is presented as rings of six carbon atoms arranged like horizontal sheets, and in this structure each carbon will be bonding to only three other carbons, this is why letter C would not be the correct answer.
Since in this molecule we only have Carbon atoms, we have a nonpolar molecule, which it is most likely to present a Covalent bond between each Carbon atom, there is no reason to have an ionic bond or metallic bond in this situation, therefore letter C is the only possible answer in this question.