हिरा _________ आहे.

Option B matches the question because Diamond is a network covalent crystal—each carbon sp³-bonded tetrahedrally in a giant lattice. Metallic, ionic, and molecular crystal types describe other bonding regimes (metals, salts, van der Waals solids) incompatible with diamond's structure. You can eliminate A (Metallic Crystal), C (Ionic Crystal), D (Molecular Crystal) — each one conflicts with the principle or calculation that leads to option B. Store the reasoning behind the answer, not just the letter, because topic packs revisit the same idea in different forms.