What is a carcinogen?

A carcinogen is defined as a substance that can lead to the development of cancer in living tissue. These substances alter cellular metabolism or damage the genetic material within cells, which can result in uncontrolled cell growth, ultimately leading to tumor formation. Knowing this, it is clear that the correct answer accurately captures the nature and role of carcinogens in biological systems.

The other options focus on unrelated biological functions; for example, promoting cellular repair refers to factors that help the body recover from damage, while enhancing photosynthesis pertains to processes in plants that use sunlight to convert carbon dioxide and water into glucose and oxygen. Aiding in digestion involves substances that help break down food into nutrients but does not pertain to cancer risk. Thus, the specificity of a carcinogen as a cancer-causing agent makes the second choice the only appropriate selection.