Please help, much appreciated

Amicroscope, because these cells are so small that you would not be able to see them, or what they attack without it.

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.

D. a  bar magnet with its south pole on the left