Answer
constructive, destructive, constructive.
Work Step by Step
At point a, it is obvious that there is a constructive interference since the two in-phase waves' crests met there.
And at point c, it is obvious that there is a constructive interference since the two in-phase waves' troughs met there.
But at point b, it is obvious that there is a destructive interference since the two in-phase waves trough met there the trough of one on the crest of the other.
