What is the name given to the phase when a new matrix and a new hair start to form?

The correct choice refers to the phase when a new matrix and a new hair begin to form, which is known as the anagen phase. During this phase, the hair bulb is actively engaged in hair production. The follicles undergo significant activity, resulting in the growth of new hair strands.

In contrast, the telogen phase is the resting phase of the hair growth cycle where the hair is not actively growing, and the follicle is simply maintaining existing hair until it eventually falls out or is shed. The catagen phase is a transitional stage where hair growth slows as the follicles shrink and prepare to enter a resting state. The exogen phase deals with the shedding of hair.

Thus, the anagen phase is specifically associated with the generation of new hair, making it the correct answer.