Two pendulum start oscillating in the same direction at the same time from the same mean position. Their time periods are respectively 2 s and 1.5 s. The phase difference between them, when the smaller pendulum has completed one vibration, will be

Smaller pendulum will be that for which time period is less, here 1.5 s. So when this completes one vibration, the bigger pendulum will complete its $\frac{3}{4}$ oscillation (because its time period is 2 s) i.e., it will be at its negative extreme position. Hence the smaller pendulum will be ahead by a phase of $\frac{\mathrm{\pi }}{2}$ than bigger pendulum at this time.