Final answer:
The structure that separates the middle ear from the inner ear is the oval window, which is option 4 in the given question.
Step-by-step explanation:
The structure that separates the middle ear from the inner ear is known as the oval window. The external ear is comprised of the auricle, ear canal, and the tympanic membrane. The middle ear houses the ossicles - malleus, incus, and stapes – and is connected to the pharynx by the Eustachian tube. Finally, the inner ear contains the cochlea and vestibule, which are responsible for audition (hearing) and equilibrium (balance), respectively.
In the scenario given, the tympanic membrane is a part of the external ear, so it cannot be the structure that separates the middle ear from the inner ear. The cochlea and Eustachian tube are also parts of the inner ear, while the tympanic cavity is essentially another term for the middle ear. Therefore, the correct answer is the oval window, option 4.