Which of the following is covalent solid?

Covalent solids form crystals that resemble a single massive molecule made up of an almost infinite number of covalent bonds.

In diamond and graphite, each carbon atom is covalently bound to four other carbon atoms oriented toward the corners of a tetrahedron. Covalent solids are often very hard and notoriously difficult to melt because all of the bonds in this structure are equally strong. Diamond is the hardest natural substance and melts at $355°\mathrm{C}$.

Hence, the required answer is B) Diamond.