Answer: Up and down
Explanation :
The waves that require a medium to travel through are mechanical waves. There are two types of mechanical waves.
- Longitudinal waves
- Transverse waves
The given figure shows a transverse wave in which the particles of the medium move up and down to the direction of propagation of waves i.e. the medium particles are perpendicular in the direction of motion of waves.
The uppermost portion of waves is called crest while the lowermost portion is a trough.
Hence, the correct option is (C).