Which of the following crystals are the hardest and most...

Diamond is a covalent network solid in which each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral geometry. This creates a rigid three-dimensional network of strong covalent bonds.

Due to this strong and highly directional bonding, diamond is the hardest known natural substance and is highly incompressible.

Magnesium oxide is an ionic solid, sugar is a molecular solid, and copper is a metallic solid, all of which are softer and more compressible than diamond.

Answer: Diamond