Concept:Carbon exists in different structural forms called allotropes, and their hardness depends on the arrangement and bonding of carbon atoms.
Explanation:Diamond is an allotrope of carbon in which each carbon atom is bonded to four other carbon atoms.
This forms a rigid, three-dimensional tetrahedral network.
The strong covalent bonds throughout this network make diamond extremely hard.
In fact, diamond is the hardest naturally occurring substance known.
Other forms like charcoal, coke, and graphite have weaker or layered structures, so they are much softer.
Answer:C. diamond