Which part of the nail structure is free and not attached to the skin?

The distal edge of the nail plate is the correct choice as it is the part of the nail that extends beyond the fingertip and is not attached to the skin underneath. This free edge is important because it allows for growth and provides structural protection to the fingertip. The design of the nail plate allows it to grow out from the nail matrix at the proximal end while the distal edge remains unattached.

Other components of the nail structure play specific roles; for instance, the nail bed is the skin beneath the nail plate, and it is firmly attached to the nail. The proximal edge of the nail plate, often referred to as the cuticle area, is where the nail is anchored to the skin at the base, while the lunula is the visible part of the nail matrix but also situated under the skin at the base. Thus, understanding the anatomy of the nail helps to clarify which part is free and plays a role in nail health and aesthetics.