Sound waves cant travel through

a. an elastic material
b. a solid
c. a vacuum
d. a liquid

C. A vacuum

Explanation:

Sound waves require a medium to travel through. Such a medium could be solid, a liquid, or a gas. We all know that sound can travel through air, and if you've ever heard a sounds through a wall or underwater you'll have experienced sound travelling through a solid and a liquid.  vacum, however, is a lack of any particles, which means the sound waves have no medium to travel through. Therefore, sound waves are not able to travel through a vacuum, as there is nothing to travel through.  

c)as speed changes, wavelength and frequency change.

everything changes!