What darkens pink melanin, causing it to turn brown?

The process that causes pink melanin to darken and turn brown is primarily influenced by exposure to ultraviolet (UV) radiation from the sun, but in the context of the options provided, oxygen plays a significant role in the body's reaction to UV light. When melanin is produced in response to increased UV exposure, the melanin darkens as it oxidizes in the presence of oxygen. This oxidation process helps the body protect itself from further UV damage, which is a key function of melanin.

While vitamin D and nutrients are important for skin health and other bodily functions, they do not directly cause melanin to darken. Heat, on the other hand, can cause some changes in skin pigmentation but is not specifically responsible for the oxidation process that darkens melanin. Therefore, oxygen is the most accurate choice in relation to the actual biochemical process involved in the darkening of pink melanin to brown.