Final answer:
The delayed tanning phase is induced mostly by UVB exposure and becomes visible after 72 hours. Tanning is a defense mechanism where the body produces pigments to absorb future UV exposures. UV-B radiation distorts DNA helix and can lead to mutations and possible formation of cancerous cells.
Step-by-step explanation:
Tanning is a defense mechanism in which the body produces pigments to absorb future exposures in inert skin layers above living cells. UV-B radiation excites DNA molecules, distorting the DNA helix, leading to mutations and the possible formation of cancerous cells. The phase in the tanning process that is induced mostly by UVB exposure, and becomes visible 72 hours after exposure, is called the delayed tanning phase.