What principle states that energy must be absorbed by an object to produce an effect?

The correct answer is based on the Grotthuss-Draper law, which primarily states that only the radiation that is absorbed by an object can produce a physiological effect. This principle is crucial in fields such as photobiology and is applied in various technologies, including laser therapies. The idea here is that for there to be any observable effect, such as heating or stimulating a biological response, the energy in the form of light or radiation must first be absorbed by the subject or tissue.

Other concepts listed do not specifically articulate this relationship between absorbed energy and its effect. The laws of thermodynamics pertain broadly to energy transfer and transformations but do not directly address the specific process of absorption leading to effect. The photosynthesis principle relates to how plants convert light energy into chemical energy, which is a different subject. Optical density theory deals with how light is transmitted through materials and the absorption properties of those materials but does not emphasize the necessity of absorption for producing a physiological effect. Thus, the Grotthuss-Draper law is specifically targeted to this absorption-effect relationship.