Final answer:
Sound waves require a medium to travel and cannot be heard in a vacuum, which lacks particles to facilitate sound transmission.
Step-by-step explanation:
Sound is a type of mechanical wave that requires a medium to travel through. Among the options provided, sound cannot be heard in a vacuum. This is because a vacuum lacks a medium such as air, water, or solid matter for sound waves to travel through. Sound waves need particles to compress and transmit the energy from one particle to the next, and in a vacuum, there are no particles to facilitate this transmission. Therefore, the correct answer is (c) vacuum. While sound travels slower in wet cement compared to air or water due to the cement's density, it can still be heard to some extent. Similarly, sound can travel through air and water, though the speed of sound varies depending on the medium's density and elasticity.