Constructive Interference occurs when the crust of one way falls on the crust of other and the trough of one wave falls on the trough of other. This way the points of the wave re-enforce each other resulting in an increased amplitude of the wave.
Since the crust and troughs of two waves are at the same place at the same time, the waves are said to be in phase. So the correct answer to the question is option A.
Light waves have constructive interference because they are emitted In Phase
