I think that the best answer is first one:
a) Trace the path that the sound waves travel as they enter the ear and proceed to the receptor cells for hearing, making sure to explain the process of transduction.
A. It has mass.
Air is matter, and can't be seen thus not all matter can be seen.
Any liquid or gaseous is matter (water, air, etc.)
Since it can be liquid or gaseous it doesn't have to maintain it's shape and size to be matter.
