Which of the following structures is responsible for nail growth?

The nail matrix is the correct answer because it is the area of actively dividing cells located beneath the base of the nail, just under the cuticle. This matrix is responsible for producing new nail cells, which harden and form the visible nail plate as they push forward. As new cells are formed, they cause the nail to grow longer.

Understanding the function of the nail matrix is crucial for anyone in cosmetology, as proper care and attention to this area can impact nail health and growth. Techniques used in manicures and nail treatments often focus on promoting the health of the nail matrix to facilitate optimal nail growth. The other structures mentioned, while related to the nail, do not contribute directly to the growth process like the nail matrix does.