Which area is not always visible on the nail bed?

The lunula is considered the part of the nail bed that is not always visible. It is the small, crescent-shaped area located at the base of the nail, just above the cuticle. While the lunula is visible on some people's nails, it can be entirely covered or less pronounced on others, especially if the nail is short or the cuticles are not pushed back.

In contrast, other components like the hyponychium, which is located beneath the free edge of the nail plate and protects the nail bed, and the perionychium, the skin surrounding the nail, are generally visible regardless of the individual’s nail shape or length. The nail plate itself is the hard part of the nail that is always visible and extends from the cuticle to the fingertip. Therefore, the lunula is distinct in that its visibility varies among individuals, making it the correct choice for the question asked.