The membranous labyrinth contains

The correct option is Endolymph.

To understand why this is the correct answer, let's break down the anatomy of the inner ear:
The inner ear is composed of two main parts:
1. The Bony Labyrinth: This is the outer, hard bony shell of the inner ear.
2. The Membranous Labyrinth: This is a system of membranous ducts and sacs situated inside the bony labyrinth.

These two compartments contain different types of fluid:
- The space between the outer bony labyrinth and the inner membranous labyrinth is filled with a fluid called perilymph. Perilymph is chemically similar to cerebrospinal fluid and extracellular fluid (high in sodium ions, low in potassium ions).
- The interior of the membranous labyrinth itself is filled with a different, specialized fluid called endolymph. Endolymph is unique because it has a high concentration of potassium ions and a low concentration of sodium ions, which is crucial for generating the electrical signals needed for hearing and balance.

Therefore, the membranous labyrinth contains endolymph, making it the correct option.