The softest rock is talc. It is a mineral that is composed of magnesium, silicon, and oxygen. It is a very soft mineral and can be scratched with a fingernail. It is also a very slippery mineral and is often used in talcum powder.
Marble is a metamorphic rock that is composed of calcium carbonate. It is a hard rock and can be scratched with a knife. It is often used in construction and for making sculptures.
Diamond is a gemstone that is composed of carbon. It is the hardest natural substance on Earth. It can only be scratched by another diamond. It is often used in jewelry.
Quartz is a mineral that is composed of silicon dioxide. It is a hard rock and can be scratched with a knife. It is often used in jewelry and for making glass.
Therefore, the answer to the question “The softest rock is” is C. Talc.