(N/A) Ear ossicles: The middle ear contains three tiny bones called malleus,incus,and stapes,collectively known as ear ossicles. They are attached to one another in a chain-like fashion and increase the efficiency of transmission of sound waves to the inner ear.
$(b)$ Cochlea: The cochlea is the coiled portion of the labyrinth of the inner ear. It is filled with fluid and contains the sensory receptors for hearing.
$(c)$ Organ of Corti: It is a structure located on the basilar membrane of the cochlea. It contains hair cells that act as auditory receptors. The hair cells press against the tectorial membrane when sound waves cause vibrations in the basilar membrane,generating nerve impulses.
$(d)$ Synapse: $A$ synapse is a junction between two neurons. It is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron,which may or may not be separated by a gap called the synaptic cleft. It allows the transmission of nerve impulses from one neuron to another via chemical or electrical signals.