The hardest rock is A. Marble B. Diamond C. Talc D. Quartz

Marble is a metamorphic rock that is formed when limestone is subjected to heat and pressure. Marble is a relatively soft rock and is not as hard as diamond. It is used in construction, sculpture, and other applications where a decorative stone is desired.

Talc is a soft mineral that is used in talcum powder, cosmetics, and other applications where a lubricant or absorbent is needed. Talc is not as hard as diamond or marble.

Quartz is a hard mineral that is used in jewelry, glass, and other applications where a durable material is needed. Quartz is not as hard as diamond, but it is harder than marble and talc.

In conclusion, diamond is the hardest rock on Earth. It is a form of carbon that has been subjected to extreme pressure and heat. Diamond is used in jewelry, industrial cutting tools, and other applications where high hardness is required.