1. The vibrations which occur in pendulums B and D are called forced vibrations.
2. Pendulum C is in the state of resonance with pendulum A as it is of the same length.
3. The pendulums vibrate because the forced vibration from A is transferred due to string PQ. Pendulum B is of a different length as compared to pendulum A. Hence, it will continuously vibrate with a frequency which is different from that of pendulum A. Its amplitude will also be very small.
Pendulum C is of the same length as compared to pendulum A. Hence, it will vibrate in phase with pendulum A. Its amplitude will be equal to that of pendulum A as it will attain resonance.