Which type of wave moves perpendicular to the direction it is traveling?

A transverse wave is indeed characterized by its movement being perpendicular to the direction of travel. In a transverse wave, particles of the medium move up and down or side to side while the wave itself moves forward. This can be observed visually in waves on a string, where peaks and troughs are formed as the wave propagates along the length of the string.

In contrast, a longitudinal wave involves movement parallel to the direction of wave travel. Here, particles of the medium compress and rarefy, creating areas of high and low pressure that move along the direction of the wave's energy transfer. Examples include sound waves traveling through air.

Surface waves, which occur at the interface between two different mediums, such as water and air, exhibit characteristics of both transverse and longitudinal waves. While they do have a complex motion, they do not strictly align with the perpendicular movement described in the question.

Mechanical waves require a medium to travel through, and while they encompass both transverse and longitudinal waves, they do not specifically define the perpendicular movement criterion.

Thus, the defining feature of a transverse wave emphasizes how it uniquely fulfills the condition of movement perpendicular to its direction of travel, making it the correct answer.