The border between the middle and inner ear is formed by

The correct option is oval window.

To understand why this is correct, we can look at the anatomical structure of the human ear, which is divided into three main parts: the outer ear, the middle ear, and the inner ear.

1. Outer Ear: This includes the pinna (auricle) and the external auditory canal. It ends at the tympanic membrane (eardrum), which acts as the boundary between the outer ear and the middle ear.
2. Middle Ear: This is an air-filled cavity containing three tiny bones called auditory ossicles: the malleus (hammer), the incus (anvil), and the stapes (stirrup). These bones transmit sound vibrations from the tympanic membrane across the middle ear cavity.
3. Inner Ear: This is a fluid-filled structure containing the cochlea (responsible for hearing) and the vestibular system (responsible for balance).

The transmission of sound vibrations from the middle ear to the inner ear occurs at the interface where the stapes (the last ossicle of the middle ear) connects to the cochlea. This interface is a membrane-covered opening known as the oval window. Therefore, the oval window forms the physical and functional border between the air-filled middle ear and the fluid-filled inner ear.