A, as can be seen from the figure, if the two waves have the same wavelength and wave speed, the frequencies are equal and interference can occur. The two waves make the vibration direction of particle M the same, so it is always strengthened, so A is wrong.

B, from the time shown in the diagram, after the 14 A wave period, M will be at a low point, so B is wrong.

C, two series of harmonic shear waves propagate on the same rope, and their wavelengths are equal, so their wave velocities are equal, so C is wrong.

D, the wave velocity along the same elastic rope is equal, so D is correct.

So choose D.