From the fluctuation image, at this moment, particle A is at the peak, and the speed of particle B in the equilibrium position is negative on the Y axis, but the vibration image that is now timed is not;

In a quarter period, particle B is at the trough, and the velocity of particle A at the equilibrium position is negative for the Y axis, but the vibration image at the current moment is not;

In a half period, particle A is at the trough, and the velocity of particle B at the equilibrium position is positive on the Y axis, but the vibration image at the current moment is not;

In three-quarters of the period, particle B is at the peak, and the velocity of particle A at the equilibrium position is positive on the Y axis. Now the timed vibration image is A, so A is correct.

So choose: a.