Sound waves travel faster in: 

As is common knowledge, sound waves are longitudinal waves that may travel through gases, liquids, and vacuum. In its most basic form, the sound is transmitted through the mediation of particle collisions.
As the material through which it passes stiffens and loses density, this movement happens more quickly. The speed at which sound moves through a medium depends on its density.

In liquids, sound moves more quickly than in gases, and in solids, it moves more quickly than in liquids. It is possible to think of sound as a mechanical process in which molecules and atoms are compressed before becoming rarefied. When sound waves travel across a medium, the material inside the medium temporarily pushes against one another before re-expanding and continuing to press against the material.
As seen above, a sound wave may go through a medium more quickly the denser it is.
And according to the suggested answer above, solids have a larger density than liquids and gases, thus sound moves through them more quickly.

Hence, the correct answer is option A.