Final answer:
The characteristic of a transverse wave measured along the direction of propagation is the wavelength, which is the distance between repeating parts of the wave such as crest to crest.
Step-by-step explanation:
The characteristic of a transverse wave that is measured along the direction of propagation is the wavelength. The wavelength is defined as the distance between adjacent identical parts of the wave, such as from crest to crest or trough to trough, which runs parallel to the direction the wave is moving.
On the contrary, amplitude is the distance from the midpoint to the crest or trough of the wave and is measured perpendicular to the direction of propagation. Frequency is the number of complete cycles per second and is not dependent on the direction of propagation. Period is the time it takes for one complete cycle and, like frequency, does not require measurement along a particular direction.