Final answer:
A trough in a wave is the lowermost position of a wave. The distance between the crest and the equilibrium position is equal to the distance between the trough and the equilibrium position. Hence the correct answer is option A
Step-by-step explanation:
A trough in a wave is the lowermost position of a wave (option a). It is the point on the waveform that is furthest below the equilibrium position. The amplitude of a wave, represented by the symbol A, is the characteristic height of the wave. The distance between the crest and the equilibrium position is equal to the distance between the trough and the equilibrium position.