The correct answer is: B. Tetrahedral structure of diamond.
Diamond is a crystalline allotrope of carbon with a diamond cubic crystal structure. It is the hardest naturally occurring substance known, and is the most widely used gemstone. Graphite is another allotrope of carbon, but it has a layered structure. The layers of graphite are held together by weak van der Waals forces, which means that they can slide past each other easily. This makes graphite soft and slippery.
In contrast, the tetrahedral structure of diamond makes it very strong and hard. The carbon atoms in diamond are bonded together in a tetrahedral arrangement, which means that each carbon atom is bonded to four other carbon atoms. This strong bonding makes diamond very difficult to break or scratch.
The difference in the crystalline structures of diamond and graphite is the main reason why diamond is harder than graphite.