option c...have the waves move thru the vacuum or space
Constant speed along the inclined surface = 30 m / 4 s = 7.5 m/s
Vertical speed = inclined speed * sin(45) = 7.5 *√2 / 2 = 5.3 m/s
Answer: 5.3 m/s
If you put it into English in the comments I would be more then happy to help you! Thank you!