Which is true of a mechanical wave?

a) it must have a medium.

b) it must travel in empty space.

c) both sound and light are examples.

d) it can travel in a vacuum.

A) True

Mechanical waves need a medium to travel.

B) False.

Mechanical waves need a medium to travel. So they can not travel in empty space.

C) False

Sound is a mechanical wave since it needs a medium to travel. light wave is electromagnetic wave and it can travel in empty space too.

D) False

Mechanical waves need a medium to travel. So they can not travel in empty space.

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.