For a covalent solid, the units which occupy lattice points are

A covalent network crystal is made up of atoms at the crystal's lattice points, with each atom covalently bonded to its nearest neighbor atoms. The three-dimensional covalently bonded network is densely packed with atoms. Graphite and diamond are two examples of these crystals. Atoms in these crystals form covalent bonds with one another.

Hence, the required answer is A) Atoms.