**Core Concept**
Endolymph is a unique fluid found in the membranous labyrinth of the inner ear, playing a crucial role in the vestibular and auditory functions. The composition of endolymph is distinct from perilymph, with a high concentration of potassium ions.

**Why the Correct Answer is Right**
Endolymph is secreted by the marginal cells of the stria vascularis, which are specialized epithelial cells lining the lateral wall of the cochlear duct. These cells possess a unique polarity, with a high density of Na+/K+-ATPase pumps on their basolateral membrane, allowing them to secrete potassium ions into the endolymph. This process is essential for maintaining the ionic balance and generating the endocochlear potential, which is necessary for sound transduction.

**Why Each Wrong Option is Incorrect**
**Option A:** The Reissner's membrane, which separates the scala media from the scala vestibuli, is not responsible for endolymph secretion.
**Option B:** The stellate cells of the cochlear duct are involved in the regulation of potassium ions but are not the primary source of endolymph secretion.
**Option C:** The saccular duct, part of the membranous labyrinth, is involved in the vestibular system but is not responsible for endolymph secretion.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the stria vascularis is sensitive to ototoxic agents, which can disrupt endolymph secretion and lead to hearing loss or balance disorders.

**Correct Answer:** C. The stria vascularis.