Substance X melts at 20°C and boils at 50'C.
At what temp will substance x freeze?

c. it will increase in speed is the correct answer.

Explanation:

Sound waves move faster in more solid substance, and the denser the body the deeper the sound moves.The speed of sound is faster in solid substances and it is slower in fluids or gases.The more solid is material the molecules are very closer to each other and the less time it needs molecules to move.Thus if a sound wave moves from the air through a solid  it will increase in speed.