Answer: Third option; "travels slower in solids because particles are close together"
Explanation: While in the air the particles are very freely and the sound does not need to do a lot of work, in a dense material this is not the case. The particles are more close together, and this causes that in a fixed distance, the sound wave needs to move more particles in a solid than in the air. This makes the sound wave to travel slower when it is in a solid.