Concept: This question tests the understanding of the mechanism of hearing, specifically how sound vibrations are transduced into neural signals within the cochlea.

Why (A) is correct: When sound vibrations reach the fluid of the cochlea, they generate waves in the perilymph and endolymph. These fluid movements cause a ripple in the basilar membrane. The movement of the basilar membrane, in turn, causes the hair cells (stereocilia) of the Organ of Corti (which rests on the basilar membrane) to bend against the tectorial membrane, generating nerve impulses.

Why other options are wrong:

• (B) tectorial membrane: The tectorial membrane is stationary; the stereocilia of hair cells bend against it due to the movement of the basilar membrane.
• (C) reissners membrane: Reissner's membrane (vestibular membrane) separates the scala vestibuli from the scala media but is not directly involved in the ripple effect that stimulates hair cells.
• (D) tympanic membrane: The tympanic membrane (eardrum) is in the outer ear and is the first structure to vibrate in response to sound waves, not the structure that ripples within the cochlea.

NEET Tip: Remember the sequence: Sound waves → Tympanic membrane → Ossicles (Malleus, Incus, Stapes) → Oval window → Perilymph/Endolymph → Basilar membrane ripple → Hair cells bend against Tectorial membrane → Nerve impulses.

Correct Answer: (A)