What does the term 'tanning effectiveness' refer to?

The term 'tanning effectiveness' specifically relates to the color depth achieved in the skin after undergoing a tanning process. This means it measures how dark or bronzed the skin has become as a result of exposure to UV light, whether from sun exposure or artificial tanning methods. Achieving a deeper and more evenly distributed color indicates a higher level of tanning effectiveness.

The other options focus on different aspects of the tanning process. For instance, the speed of the tanning process describes how quickly a tan develops, while the safety of tanning products pertains to the potential risks and health implications of the chemicals involved. The frequency of tanning sessions required deals with how often one needs to tan to maintain or achieve the desired color. However, none of these factors directly define the effectiveness of tanning in terms of the darkness of the skin, which is why the correct answer centers on the achieved color depth.