Here's why:

* Sound needs a medium: Sound is a mechanical wave, which means it needs a medium (like air, water, or solids) to travel. It cannot travel through a vacuum, where there are no particles to vibrate.

* Speed in different mediums: Sound travels at different speeds in different mediums. It travels fastest in solids, then liquids, and slowest in gases. The speed of sound in air is approximately 343 meters per second, while the speed of sound in a vacuum is zero.

The other statements are correct:

* a) Sound travels in a straight line: Sound waves generally travel in a straight line unless they encounter an obstacle that causes them to reflect or diffract.

* b) Sound travels as a wave: Sound is a type of longitudinal wave, meaning the particles in the medium vibrate parallel to the direction the wave travels.

* c) Sound is a form of energy: Sound is a form of mechanical energy that is transmitted through vibrations.