Which of the following is a fluid-filled structure in the inner ear?

The cochlea is a fluid-filled structure in the inner ear crucial for the process of hearing. It is a spiral-shaped organ that contains the sensory cells necessary for transducing sound vibrations into neural signals. Inside the cochlea, there are three fluid-filled chambers: the scala vestibuli, scala tympani, and scala media. The fluid movement within these chambers helps stimulate hair cells located in the organ of Corti, which sends auditory information to the brain.

In contrast, the other structures listed do not have the same fluid-filled characteristics. The anvil, also known as the incus, is one of the three tiny bones in the middle ear that helps transmit sound vibrations; it's not fluid-filled. The pinna, or outer ear, is primarily made of cartilage and skin; it serves as a funnel for sound waves but does not contain fluid. The eardrum, or tympanic membrane, is a membrane that vibrates in response to sound waves, also lacking fluid. Thus, the cochlea is specifically recognized for its role in hearing through its fluid-filled anatomy, distinguishing it from the other options.