Final answer:
The correct answer to what the thin layer of tissue between the nail plate and the nail bed is known as the eponychium, also referred to as the cuticle.
Step-by-step explanation:
The thin layer of tissue between the nail plate and the nail bed is called the eponychium, also known as the cuticle. This structure is an extension of the nail fold that meets the proximal end of the nail body and serves to protect the area against infections by sealing the edges of the nail to the nail bed. The nail matrix is the part of the nail that is lodged in the deep layer of the epidermis, responsible for nail growth. It is not visible from the top as it lies underneath the nail. The lunula is the visible part of the matrix that appears as a white crescent at the base of the nail. Therefore, the correct answer is d. eponychium.