**Core Concept**
Müller's muscle is a type of smooth muscle found in the eye, specifically in the anterior segment. It plays a crucial role in the regulation of the pupil's size and the movement of the iris. Müller's muscle is innervated by the sympathetic nervous system and is responsible for the dilation of the pupil.

**Why the Correct Answer is Right**
Müller's muscle is located in the anterior ciliary muscle, which is situated near the ciliary body. When stimulated by the sympathetic nervous system, Müller's muscle contracts and pulls on the iris, causing it to dilate. This is an important mechanism for regulating light entry into the eye and is essential for adjusting to changes in lighting conditions. The contraction of Müller's muscle also helps to reduce the tension in the ciliary body, which is necessary for the accommodation of the lens.

**Why Each Wrong Option is Incorrect**
**Option A:** Müller's muscle is not found in the eyelid or the conjunctiva, which are both located in the external eye.
**Option B:** Müller's muscle is not associated with the extraocular muscles, which are responsible for the movement of the eye.
**Option C:** Müller's muscle is not found in the lacrimal gland or the accessory lacrimal gland, which produce tears.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Müller's muscle is responsible for the dilation of the pupil, and its dysfunction can lead to conditions such as Horner's syndrome, which is characterized by miosis (pupil constriction), ptosis (eyelid drooping), and anhidrosis (lack of sweating).

**Correct Answer:** C.