The correct answer is: C. Diamond
Diamond is the hardest substance available on Earth. It is a pure form of carbon that has been subjected to extreme pressure and heat. This process has caused the carbon atoms to bond together in a very strong way, making diamond the hardest natural substance known.
Gold is a soft, yellow metal that is often used in jewelry. It is not as hard as diamond, but it is very durable and does not tarnish easily.
Iron is a hard, gray metal that is often used in construction and manufacturing. It is not as hard as diamond, but it is stronger and more ductile.
Platinum is a hard, white metal that is often used in jewelry and electronics. It is not as hard as diamond, but it is more resistant to corrosion.
In conclusion, diamond is the hardest substance available on Earth. It is a pure form of carbon that has been subjected to extreme pressure and heat. This process has caused the carbon atoms to bond together in a very strong way, making diamond the hardest natural substance known.