Step 1: Understanding the Concept
For a particle in S.H.M. starting from the mean position, the phase after time \( t \) is \( \phi = \omega t = \frac{2\pi}{T}\,t \).
Step 2: Phase of A
A completes one oscillation at \( t = T \), so \( \phi_A = \frac{2\pi}{T}\times T = 2\pi \).
Step 3: Phase of B
B has period \( \frac{3T}{2} \), so \( \phi_B = \frac{2\pi}{3T/2}\times T = \frac{4\pi}{3} \).
Step 4: Difference
\( \Delta\phi = 2\pi - \frac{4\pi}{3} = \frac{2\pi}{3} \), which is option (D).
Final Answer:
The phase difference is 2pi/3.
\[ \boxed{\text{(D) }\left(\dfrac{2\pi}{3}\right)} \]