What term describes an organism's capacity to respond to light and dark periods?

The correct term that describes an organism's capacity to respond to light and dark periods is photoperiodism. Photoperiodism is a physiological reaction of organisms, particularly plants and some animals, to the length of day or night—a crucial factor in the timing of processes such as flowering, hibernation, and migration. This response helps organisms adapt to seasonal changes by optimizing their growth or reproductive strategies according to the amount of available daylight.

Circadian rhythm, while related, specifically refers to the internal biological clock that regulates the cycle of physiological processes on a roughly 24-hour cycle, influenced by light and darkness. Infradian rhythms pertain to biological cycles that take longer than 24 hours, such as menstrual cycles, and seasonality refers to changes that occur in response to different seasons but does not specifically address the direct response to light and dark.