Which layer of skin is slightly thickened and lies beneath the free edge of the nail plate?

The hyponychium is the layer of skin that is located beneath the free edge of the nail plate. It acts as a barrier to protect the nail bed from infection and foreign substances. This thickened area is important in preventing the entry of pathogens, as it adheres to the underside of the nail. The hyponychium also plays a role in the overall health of the nail by providing support and anchorage.

In contrast, the eponychium is the tissue that overlaps the nail plate at the base of the nail, serving a protective function during the growth of the nail. The cuticle consists of the dead skin that is left on the nail plate and also helps protect the area from infection but is not situated beneath the free edge. The nail bed is the living layer of tissue beneath the nail plate, but it does not specifically refer to the thickened skin at the free edge. Therefore, the hyponychium is the correct answer as it directly pertains to the thickened skin under the nail plate's free edge.