Answer: A (diamond and silicon dioxide). Diamond is a giant covalent structure in which every carbon atom is covalently bonded to four others, forming a rigid tetrahedral three-dimensional network. Silicon dioxide, SiO₂ (silica), has essentially the same arrangement: each silicon atom is bonded to four oxygen atoms in a tetrahedral 3D lattice, like diamond but with oxygen atoms alternating between the silicon atoms.
Because both are giant covalent solids built on the same tetrahedral framework, they share a similar three-dimensional arrangement of atoms, which is why they have very high melting points.
The other pairs differ. Graphite is layered, made of flat 2D sheets of hexagons, so it is not like diamond's 3D network. Iodine is a simple molecular solid of discrete I₂ molecules, not a giant 3D structure at all. The answer is A.
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