The correct answer is enamel of teeth.
Enamel is the hardest substance in the human body. It is a very hard, white, and shiny substance that covers the crown of the tooth. It is made up of minerals such as calcium and phosphorus. Enamel is very important for protecting the teeth from decay and damage.
The skull bones of the head are also very hard, but they are not as hard as enamel. The skull bones protect the brain and other important organs in the head.
Thumb nails are made of keratin, the same protein that makes up hair and skin. They are not as hard as enamel or the skull bones, but they are still very strong.
The spinal vertebra are the bones that make up the spine. They are not as hard as enamel or the skull bones, but they are still very strong. They protect the spinal cord and allow the body to move.
In conclusion, the hardest part of the human body is the enamel of the teeth. It is a very hard, white, and shiny substance that covers the crown of the tooth. It is made up of minerals such as calcium and phosphorus. Enamel is very important for protecting the teeth from decay and damage.