Which waves always require matter in order to propagate?

a. Heat waves

b. Light waves

c. Sound waves

d. Transverse waves

I believe the correct answer is sound waves. It is the sound waves that always need matter or a medium in order to propagate. Sound waves are mechanical waves that has different characteristics to electromagnetic waves which does not requires a medium to travel. Mechanical waves depend on the particles interaction for it to travel hence a medium is needed.