Explanation:

- Let the length of Train P = L meters.

- Train P crosses a pole in 30 s ? speed of P = L/30 m/s.

- Train P crosses 200 m tunnel in 40 s ? (L + 200)/40 = L/30 ? 30(L + 200) = 40L ? 30L + 6000 = 40L ? 10L = 6000 ? L = 600 meters.

- So, speed of P = 600/30 = 20 m/s = 72 kmph.

- Speed of Q is 18 kmph more = 90 kmph = 25 m/s.

- They run in opposite directions, combined speed = 20 + 25 = 45 m/s.

- They cross in 20 s: (Length of P + Length of Q)/20 = 45 ? (600 + Length of Q) = 900 ? Length of Q = 300 meters.

- Correct Option: 1, 300 meters

- All other options do not fit the calculation.

- Option 2, 3, 4, 5 are incorrect.

- The correct length of Train Q is 300 meters.