Bones of the reptiles are covered by

The correct option is Keratin.

Reptiles are characterized by having dry, scaly skin that provides protection and prevents water loss. The outer covering of their body, including the scales and the protective layers covering bony structures (such as the scutes covering a turtle's bony shell), is composed of keratin. Keratin is a tough, fibrous structural protein that is also found in human hair and nails, as well as the claws, hooves, and feathers of other animals.

Let's examine why the other options are incorrect:
- Chitin: This is a fibrous substance consisting of polysaccharides that forms the exoskeleton of arthropods (such as insects and crabs) and the cell walls of fungi.
- Calcium: Calcium is an essential mineral that makes up the internal matrix of bones to provide structural strength, but it does not serve as an outer covering layer.