What defines the amplitude of a wave?

The amplitude of a wave is defined as the maximum distance that the wave travels from its equilibrium position, measured from either the crest or the trough to that central position. This definition captures the energy of the wave; a larger amplitude indicates that the wave carries more energy. Therefore, option B correctly identifies the amplitude as the distance from the crest (or trough) to the equilibrium position.

The other options do not accurately represent the concept of amplitude. For instance, the distance from trough to crest will provide the total height of the wave but does not denote the amplitude itself, as it measures the entire span rather than the distance from the midpoint. The reference to the distance from one wave to another does not relate to amplitude but rather describes the wavelength. Lastly, while the "height of the wave" sounds similar to amplitude, it is not specific enough to convey that the height is measured relative to the equilibrium position, which is essential for defining amplitude correctly.