Which variable is represented by the following symbol? ∑F a. Normal Force
b. Applied Force
c. Weight
d. Net Force

The variable represented by that symbol is: d. Net force.

Sounds propagates through some kind of medium. usually it's air, but it goes just as well through solid objects like walls, windows etc. a soundwave will compress the (air)molecules in front of it, and if this increased density wave hits your ears, you register it as sound. solid objects like walls are a lot denser than air, this means that they will damp a wave, but also carry it easier.