A, the particle P vibrates upwards. According to the "uphill and downhill method", it is known that the propagation direction of the wave is along the positive direction of the X axis, so A is correct.

B, the distance that a particle vibrates in a period is equal to four times the amplitude, that is, 20cm ... so, B is wrong.

C, P and Q are separated by a wavelength, and the vibration steps are the same, so C is wrong.

D, because the wavelength is equal to 0.4m, then t = λ v = 0.42s = 0.2s, knowing that t=0. 15 s, the particle P is above the equilibrium position and vibrates upwards. So, d is wrong.

So choose a.