Answer: The correct answer is "surface wave".
Step-by-step explanation:
There are three types of the mechanical wave: Longitudinal wave, Transverse wave and Surface wave.
Longitudinal wave: In this wave, the motion of the particles is parallel to the direction of the propagation of the wave. For example, sound wave.
Transverse wave: In this wave, the motion of the particles is perpendicular to the direction of the propagation of the wave. For example, light wave.
Surface wave: In this wave, the motion of the particles is neither parallel nor perpendicular to the motion of the wave. It moves in a circular motion.
Therefore, a pebble drops straight down into a tub of water, setting off surface waves that travel at the boundary between the water and air.