Sound waves can't travel through
a. an elastic material.
b. a liquid.
(c. a solid.)
d. a vacuum.
i answered c and it's not correct so i dont know what else it could be

Sound waves can travel through ANY material ... solid, liquid, or gas.
The thing is: Sound NEEDS material to travel through, and if there's
nothing there, then sound can't go anywhere.  The correct choice
on the list is 'D' ... Sound can't travel through vacuum, because
there's nothing there.  When we talk to each other, the AIR between
us carries the sound waves.

Have you ever seen any video of two astronauts standing on the
moon, or two astronauts "space-walking" on the outside of the
Space Shuttle ?

Naturally, they have their helmets on, and they're talking to
each other.  How are they talking ?  No matter how close together
they are, they're talking to each other by radio ! because there's
nothing between them to carry sound.  Even if they're only a few
inches apart, they can SHOUT but the other one can't hear it.

I have read ... I don't think there's any video of this ... that sometimes
when two astronauts are very close together, they can talk if they
touch their helmets together.  Then the sound vibrations can go from
one helmet over to the other one, through the solid shell of the helmets.

Amplify and switch electrical power and electronic signals