**Core Concept**
The telogen phase is a part of the hair growth cycle, characterized by a resting state of the hair follicle. This phase is crucial for the regulation of hair growth, and its duration is influenced by various factors, including hormonal imbalances and nutritional deficiencies.

**Why the Correct Answer is Right**
The telogen phase typically lasts for 3 months (approximately 100 days). During this time, the hair follicle is in a dormant state, and the hair is not actively growing. At the end of the telogen phase, the hair follicle enters the anagen phase, where new hair growth begins. The transition from telogen to anagen is triggered by the release of certain hormones, such as cortisol and thyroid hormones, which stimulate the hair follicle to produce a new hair shaft.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the telogen phase is shorter than the anagen phase, which can last from 2 to 6 years.
* **Option B:** This option is incorrect because the catagen phase is a brief transitional phase between the anagen and telogen phases, lasting only about 2-3 weeks.
* **Option C:** This option is incorrect because the anagen phase is the active growth phase of hair growth, not the resting phase.

**Clinical Pearl / High-Yield Fact**
It's essential to note that hormonal imbalances, particularly those affecting the thyroid gland, can disrupt the normal hair growth cycle, leading to conditions such as telogen effluvium, where there is excessive hair shedding.

**Correct Answer:** D. 3 months