The sends sound vibrations to the inner ear.
a. malleus
b. stapes
c. incus
d. tympanum

The answer is letter b. stapes.
>The malleus is the outermost and largest of the three small bones in the middle ear, and reaches an average length of about eight millimeters in the typical adult.
>The stapes is a bone in the middle ear of humans and other mammals  involved in the conduction of sound vibrations to the inner ear.
>The incus the middle one of a chain of three small bones in the middle ear of humans and other mammals
>The tympanum is an external hearing structure in animals such as frogs

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