Nails are hard, dead cells which have been converted to what?

Nails are primarily made up of a protein called keratin, which is a tough, fibrous protein that provides strength and resilience. Keratin is the same substance that forms hair and the outer layer of skin, serving as a protective barrier. When cells in the nail matrix divide and harden, they produce keratin, which eventually forms the hard structure of the nail. This process is crucial for the overall health and durability of nails, allowing them to protect the fingertips and enhance the ability to grip objects.

The other options do not accurately describe the material that makes up nails. Collagen is a protein that forms connective tissues, while Vitamin D is a vital nutrient important for bone health. Myelin is a lipid-rich substance that forms a sheath around nerves, facilitating the conduction of electrical impulses. None of these substances relate to the composition of nails, which is why keratin is the correct answer.