How do indoor tanning products modify the skin's response to UV exposure?

Indoor tanning products primarily modify the skin's response to UV exposure by enhancing melanin production. When these products are applied to the skin, they typically contain ingredients designed to stimulate the production of melanin, the pigment responsible for the color of the skin and the body’s natural defense against UV radiation. Increased melanin levels can lead to a darker tan, offering some degree of protection against UV damage.

This enhancement of melanin production is important because melanin absorbs UV radiation, which helps reduce its harmful effects. By boosting the skin's natural tanning process, indoor tanning products can promote a tanned appearance while also offering some level of protection from over-exposure to UV light.

The other options do not accurately describe the function of indoor tanning products. While blocking UV entirely would imply complete protection—which is not the case, as most indoor tanning products merely enhance natural tanning without providing full protection—cooling the skin or increasing water content does not relate to how these products affect melanin production and the skin's ability to respond to UV light.