**Core Concept**
The scala tympani and vestibuli are compartments within the cochlea and vestibule of the inner ear, respectively, involved in sound processing and balance. The drainage of these compartments is crucial for maintaining the delicate environment within the inner ear.

**Why the Correct Answer is Right**
The correct answer involves understanding the anatomy of the inner ear and its drainage system. The scala tympani and vestibuli are drained by the endolymphatic sac, which is a specialized structure that allows for the clearance of endolymph from these compartments. The endolymphatic sac is located in the posterior fossa and is connected to the scala tympani and vestibuli by the endolymphatic duct. This drainage system is essential for maintaining the proper ionic balance and pressure within the inner ear.

**Why Each Wrong Option is Incorrect**
* **Option A:** The cochlear aqueduct is involved in the drainage of cerebrospinal fluid (CSF) from the subarachnoid space to the inner ear, but it is not responsible for draining the scala tympani and vestibuli.
* **Option B:** The endolymphatic sac is the correct answer, but this option is missing.
* **Option C:** The labyrinthine artery supplies blood to the inner ear, but it is not involved in the drainage of the scala tympani and vestibuli.

**Clinical Pearl / High-Yield Fact**
The endolymphatic sac plays a crucial role in the maintenance of inner ear function, and abnormalities in this structure can lead to inner ear disorders, such as Meniere's disease.

**Correct Answer:** B. Endolymphatic sac.