The correct answer is option 3: Enamel of teeth.

Let`s analyze each option to understand why enamel of teeth is considered the hardest part of our body:

1. Skull bones of head: While the skull bones are indeed hard, they are not the hardest part of our body. They provide protection to the brain, but they are not as hard as enamel.

2. Thumb nails: Thumb nails are made of a protein called keratin, which is not as hard as enamel. Nails can be brittle and can easily break, so they are not the hardest part of our body.

3. Enamel of teeth: Enamel is the outermost layer of our teeth and is considered the hardest tissue in our body. It protects the inner layers of the tooth from damage while we chew and bite. Enamel is composed of a dense structure of crystals, mainly hydroxyapatite, which gives it its hardness.

4. Spinal vertebra: While spinal vertebrae are strong and provide support to the spine, they are not as hard as enamel. The bones in the spine are also designed to be flexible for movement, so they are not the hardest part of our body.

In conclusion, the enamel of teeth is the hardest part of our body due to its