Which is an opening that leads from the middle ear to the vestibule of the inner ear?

The oval window (or fenestra vestibuli) is a membrane-covered opening that leads from the middle ear to the vestibule of the inner ear.

Vibrations that contact the tympanic membrane travel through the three ossicles and into the inner ear. The oval window is the intersection of the middle ear with the inner ear and is directly contacted by the stapes; by the time vibrations reach the oval window, they have been amplified over 10 times from what they were when they contacted the tympanic membrane, a testament to the amplifying power of the middle ear.