Answer:
Constructive interference.
Step-by-step explanation:
Constructive interference is the interference which occurs when the peak of the two waves add together and the condition is that these two waves are in same phase, so that the resulting wave amplitude is equal to the addition of the individual amplitudes.
Crest is the highest point of the wave. Trough is defined as the lowest amplitude of the wave. The occurrence of constructive interference will occurs when the peaks of the two waves meet and overlap which is called the alignment of crest and trough.