Which visible part of the matrix extends from underneath the living skin?

The lunula is the correct answer because it represents the visible part of the matrix that is situated at the base of the nail, just under the skin. It appears as a semi-circular white area and is the only part of the matrix that is visible, indicating the growth area of the nail. The matrix itself is responsible for producing new nail cells, which harden and form the nail plate.

In contrast, the nail grooves are the indentations along the side of the nail that help guide the nail as it grows. The nail plate is the hard part of the nail that is visible and extends beyond the fingertip, but it is not a part of the matrix itself. The natural nail refers to the complete structure of the nail including the nail plate and matrix, but it does not specifically denote the visible part of the matrix that extends under the skin. Therefore, the lunula is the distinct visible part of the matrix that aligns with the question's focus on what is seen under the skin.