Which term describes organisms that are active during the daytime?

The term that describes organisms that are active during the daytime is "diurnal." Diurnal organisms have adapted their behaviors and physiological processes to be most active when the sun is up, which influences their foraging, mating, and social interactions. This activity pattern allows them to take advantage of daylight for visibility and temperature conditions, often enhancing their chances for survival and reproduction.

In contrast, nocturnal organisms are active at night, taking advantage of cooler temperatures and reduced competition for resources, while crepuscular organisms are active during twilight hours—dawn and dusk—often blending behaviors of both diurnal and nocturnal species. Endemic refers to species that are native to and restricted to a particular geographical area, which relates more to the distribution of species rather than their activity patterns. Thus, "diurnal" is correctly identified as the term for daytime-active organisms.