Which structure transmits signals from the brain to the cochlea?

The correct answer is that the structure responsible for transmitting signals from the brain to the cochlea is the efferent fiber. Efferent fibers originate from the auditory nervous system and convey information from the brain back to the cochlea, modulating its function. This communication is essential for various processes, such as regulating the sensitivity of hair cells and protecting against excessive sound input.

In contrast, afferent fibers transmit signals from the cochlea to the brain, allowing the brain to interpret sound. The vestibular system is involved in balance and spatial orientation but is not directly responsible for auditory signal transmission between the brain and cochlea. The cochlear duct is a part of the inner ear's structure that contains the fluid and cells necessary for hearing but does not facilitate the transmission of signals in the direction specified by the question. Thus, the efferent fibers play a crucial role in communication from the brain to the cochlea, making it the correct answer.