What is the outer-most skin layer where tanning occurs known as?

The outer-most skin layer where tanning occurs is known as the epidermis. This is the top layer of skin that acts as a protective barrier against environmental factors and is made up of several sub-layers. When skin is exposed to ultraviolet (UV) radiation from tanning beds or the sun, the cells in the epidermis, particularly the melanocytes, produce more melanin. This increased production of melanin results in the darkening of the skin, which is commonly referred to as tanning.

The dermis, which lies beneath the epidermis, contains blood vessels, nerves, and connective tissue, but it is not where the tanning reaction occurs. The hypodermis and subcutaneous layer are deeper layers that consist mainly of fat and connective tissue and also do not play a role in the tanning process itself. Therefore, the epidermis is the correct choice because it is the specific layer where the tanning response is initiated through melanin production in reaction to UV exposure.