If a crests of two waves line up and double the height of the wave, they are said to be interfering constructively.
When two waves come across each other, they get superimposed. When two waves are super imposed, the resulting wave is the sum of the individual waves at that point. Sometimes, the waves are in phase with each other, i.e the crests and troughs of the waves coincide. When this happens, the resulting wave has a height that is double the height of the individual waves.
From this we can see that the correct answer for this question is (B), constructive interference is occurring.
