During which phase do hair follicles become inactive?

The telogen phase is the correct answer because it is the final stage of the hair growth cycle, during which hair follicles become inactive, leading to the shedding of hair. During this phase, the hair is no longer actively growing, and the hair strand is anchored in the follicle but is detached from the blood supply. This inactivity can last for several weeks to months, at the end of which the hair will be shed, and a new anagen phase can begin.

In contrast, the anagen phase is the active growth stage, where the hair follicles are fully engaged in producing new hair. The catagen phase represents a transitional period where hair growth slows, and hair follicles shrink, but they are still somewhat active. The exogen phase involves the shedding of hair that has completed its cycle, which may occur at the end of either a telogen or an anagen phase but still signifies that the follicle is releasing the hair rather than being inactive. Therefore, the inactivity specific to hair follicles during the telogen phase marks it as the correct choice for this question.