What hard, tough and shiny substance covers the outer part of the teeth

To solve this question, the user needs to know about the composition of teeth and the different layers of it.

The outermost layer of the teeth is called the enamel. Enamel is a hard, tough, and shiny substance that covers the outer part of the teeth.

Now, let's go through each option and explain why it is right or wrong:

A. Wax: This option is incorrect because wax does not cover the outer part of the teeth. Wax is a sticky substance that is used for various purposes such as making candles, sealing envelopes, or polishing furniture.

B. Diamond: This option is incorrect because diamonds do not cover the outer part of the teeth. Diamonds are a precious stone that is used for making jewelry and cutting tools.

C. Keratin: This option is incorrect because keratin does not cover the outer part of the teeth. Keratin is a fibrous protein that is found in hair, nails, and skin.

D. Enamel: This option is correct. Enamel is a hard, tough, and shiny substance that covers the outer part of the teeth, protecting it from damage and decay.

Therefore, the answer is: D. Enamel