The correct answer is A. diamond.
Diamond is the hardest natural substance on Earth. It is a form of carbon that has been subjected to extreme pressure and heat. This has caused the carbon atoms to arrange themselves in a tetrahedral structure, which is the strongest possible arrangement. Diamonds are also very dense, which makes them even harder.
Iron is a metal that is often used in construction and manufacturing. It is strong and durable, but it is not as hard as diamond. Lead is a soft metal that is often used in batteries and ammunition. Copper is a metal that is often used in electrical wiring and plumbing. It is not as hard as diamond, but it is more ductile than iron or lead.
In conclusion, diamond is the hardest natural substance on Earth. It is a form of carbon that has been subjected to extreme pressure and heat. This has caused the carbon atoms to arrange themselves in a tetrahedral structure, which is the strongest possible arrangement. Diamonds are also very dense, which makes them even harder.