## Core Concept
The fixation reflex, also known as the fixation response, is a critical aspect of visual development in infants. It refers to the ability of an infant to fixate on an object or a light source. This reflex is essential for the development of normal vision. The critical age for the development of this reflex is crucial because it helps in diagnosing and managing visual impairments early in life.

## Why the Correct Answer is Right
The fixation reflex typically develops by 6 months of age. This reflex is vital for the normal development of vision, as it allows infants to focus on objects and people, which is necessary for visual acuity development. By 6 months, infants should be able to fixate on objects and track them, which is a significant milestone in visual development.

## Why Each Wrong Option is Incorrect
* **Option A:** 3 months - While infants begin to show some visual interest and can fixate on very bright lights or high-contrast objects, the critical age for a well-developed fixation reflex is a bit later.
* **Option B:** 9 months - Although significant visual development occurs by this age, it is later than the critical period for the initial development of the fixation reflex.
* **Option D:** 12 months - This is considered late for the initial development of a fixation reflex; most infants should have developed this reflex earlier.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the absence of a fixation reflex by 6 months of age can indicate a significant visual impairment or developmental delay. Pediatricians and ophthalmologists often assess this reflex during routine check-ups to monitor visual development.

## Correct Answer: C. 6 months.