**Core Concept**
The inner ear is a complex sensory organ responsible for detecting sound and maintaining balance. It is composed of the cochlea, which is involved in hearing, and the vestibular system, which is responsible for balance. Endolymph is a unique fluid found within the membranous labyrinth of the inner ear, playing a crucial role in the functioning of the vestibular system.

**Why the Correct Answer is Right**
Endolymph is a potassium-rich fluid that fills the membranous labyrinth of the inner ear. It is secreted by the stria vascularis, a specialized epithelial layer in the lateral wall of the cochlea and the vestibular system. The endolymph is in contact with the hair cells, which are sensory receptors responsible for detecting movement and changes in head position. The unique composition of endolymph allows for the detection of even slight changes in movement, enabling the vestibular system to maintain balance and orientation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not related to the inner ear or endolymph.
**Option B:** This option is incorrect because it refers to a different fluid found in the inner ear, not endolymph.
**Option C:** This option is incorrect because it is a structure within the eye, not related to the inner ear or endolymph.

**Clinical Pearl / High-Yield Fact**
The unique composition of endolymph is essential for the functioning of the vestibular system. A disorder of the endolymph, such as Meniere's disease, can lead to a range of symptoms including vertigo, tinnitus, hearing loss, and a feeling of fullness in the ear.

**Correct Answer: C. Endolymph is seen in the membranous labyrinth of the inner ear.**