Constructive interference occurs when waves meet.
a. trough to crest and crest to trough
b. crest to trough and crest to trough
c. trough to crest and trough to crest
d. crest to crest and trough to trough

Constructive interference occurs when the crest of one wave meets the crest of another wave and hence, a higher amplitude wave is formed.

Option D

Explanation:

According to the superposition principle, a constructive interference occurs when more than two waves of same kind meets at the same points, then the resultant wave amplitudes are equal to vector sums of the amplitude of all the individual waves.

When crest of a wave meets the crests of another wave, or likewise, the trough or a wave meets the trough of other wave, a higher amplitude wave is formed which is known as a constructive wave. And, when a crest of a wave reaches the trough, a lower amplitude wave is formed which is known as destructive wave.

probably not if net force is zero