In which part of the ear is the sound wave converted into electrical impulse

The correct answer would be cochlea.

The cochlea is a part of an inner ear and serves the function of hearing in an organism.

The perilymph (watery liquid) present in the cochlea moves in response to the vibrations made by the sound waves.

The movement of cochlea leads to the generation of movement in the organ of Corti and basilar membrane.

Then stereocilia present in the hair cells help in mechanosensing these movements and convert these into electrical signals.

These signals are then passed to the brain through auditory nerves for further processing.