b
(b) $\frac{{5T}}{4} = T + \frac{T}{4}$
By the time, the bigger pendulum makes one full oscillation, the smaller pendulum will make $\left( {1 + \frac{1}{4}} \right)$ oscillation. The bigger pendulum will be in the mean position and the smaller one will be in the positive extreme position. Thus, phase difference $= 90°$