Which of the following is a covalent crystal?
A. Rock Salt
B. Ice
C. Quartz
D. None

Hint: There are many types of bonding through which the elements and compounds interact in the chemical world namely the ionic bond, the covalent bond, etc. The ionic bonds are usually stronger than that of covalent bonds but in the form of the networked covalent molecule the compound is pretty strong in comparison.

Complete step by step answer:
The ionic bond is much stronger than the covalent bonds in terms of strength but the covalent bonds when exhibit and increased strength in the form of crystal occur in the form of a network usually.
The compounds that form covalent bonds usually tend to form a network of bonds to achieve strength and stability.
These are characterized as hard and brittle compounds like silicon carbide or diamond. These compounds consist of a large number of atoms of the constituent elements and which are connected in a network of chemical covalent bonds.
Rock salt is sodium chloride as it possesses ionic bonds, while ice is crystallized water and thus is made up of hydrogen bonding as a means of crystal formation.
Quarts or silicon dioxide also called silica is made up of a giant network of covalent bonds. Each silicon atom is covalently bonded with four other oxygen atoms. These atoms are in turn bonded to two other silicon atoms and thus forms a covalent network of bonds. The compound is represented by the formula $Si{O_2}$.

So, the correct answer is Option C.

Note: Quartz is related to the quaternary structure of silicon dioxide. The structure of quartz has the angle of $56^\circ $ at the edges. A hydrogen bond is a special bond that works with the compound having hydrogen. It is an electrostatic attraction between the hydrogen atoms which is covalently bound to more electronegative atoms.