Why does sound move fastest in solid mediums?

The particles are closest together in solids. Think about water. You can pass your hand through steam because there is a lot of space between the particles. Liquid water is a little harder to pass through, but it can be done. The particles in ice, however, are so close together that an object cannot pass through it.

Mechanical waves travel fastest through solids and slowest through gases.

Therefore the answer is solids.

The particles are closest to-gether in a solid formation, so waves travel further as they bump into each other sooner.