Answer: B. Diamond and silicon dioxide (quartz) are both giant covalent structures. Every valence electron is locked into a localised covalent bond — no mobile electrons exist to carry current, so both substances are electrical insulators.
A is wrong: in diamond each C is bonded to 4 other carbons (tetrahedral); in \(\text{SiO}_2\) each Si is bonded to 4 oxygens — not three.
C states the right conclusion (insulator) but the wrong reason. Electrons not the atoms are what carry electrical current; the atoms in any solid don't move, yet metals still conduct (via delocalised electrons). The cause of insulation is the absence of mobile electrons, not the immobility of the atoms.
D describes a layered structure with weak inter-layer forces — that fits graphite, not diamond or quartz. Diamond and \(\text{SiO}_2\) are 3-D networks, not layered.
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