Final answer:
The free edge (option D) is the part of the nail known as the hard margin that extends beyond the finger or toe. It is composed mainly of keratin and grows continuously from the nail matrix.
Step-by-step explanation:
The hard margin formed in the nail is known as the free edge. This margin is the portion of the nail that extends past the finger or toe's distal end. It is the part of the nail that can be trimmed or filed when caring for nails. The nail anatomy includes several parts and associated structures, such as the nail root, plate, and free margin.
Nails are comprised mainly of a protein called keratin. Continuous nail growth is facilitated by the nail matrix, which is a deep layer of living epidermal tissue containing stem cells that produce keratinocytes. The lunula, nail bed, cuticle, and hyponychium are additional structures contributing to the nail's structure and health.