[January 2013 WJEC] Points that are In Phase with Eachother - Explanation MarksWatch
B is at the same point as A on the wavefront and must be in phase with A.
D is at the corresponding point on the next wavefront and so is in phase with A. (It's a 360 degree phase shift which puts it back in phase with A.
C is less than one whole wave ahead of A and must be out of phase as the pahase angle will be greater than 0 but less than 360.