**Core Concept**
The phenomenon being referred to is telogen effluvium, a form of non-scarring alopecia characterized by diffuse hair loss. This condition is triggered by physiological stress, which disrupts the normal hair growth cycle. The stress causes a higher percentage of hair follicles to enter the telogen phase, resulting in excessive hair loss.

**Why the Correct Answer is Right**
Telogen effluvium occurs 3-4 months after the onset of stress. This delay is due to the hair growth cycle, which consists of anagen (growth), catagen (transitional), and telogen (resting) phases. The anagen phase lasts for approximately 2-3 years, during which the hair grows. After the anagen phase, the hair enters the catagen phase, which lasts for about 2-3 weeks. Finally, the hair enters the telogen phase, which lasts for approximately 3 months. The hair is then shed, and a new hair grows from the same follicle. Since the hair growth cycle is approximately 3 months long, it takes around 3-4 months for the hair loss to become apparent after the onset of stress.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the time period for hair loss to become apparent after the onset of stress is not instantaneous or immediate.
* **Option B:** This option is incorrect because the time period for hair loss to become apparent after the onset of stress is not as short as 1-2 months.
* **Option D:** This option is incorrect because the time period for hair loss to become apparent after the onset of stress is not as long as 6-12 months.

**Clinical Pearl / High-Yield Fact**
Telogen effluvium is a common cause of diffuse hair loss, and it is often triggered by physiological stress, such as severe illness, major surgery, or significant weight loss. It is essential to diagnose and treat the underlying cause of stress to prevent further hair loss.

**Correct Answer:** C. 3-4 months.