Explanation:

- To find the length of train A, we use the formula: Speed × Time = Distance.

- Train A crosses a platform in 30 seconds at 20 m/s. So, length of train A + length of platform = 20 m/s × 30 s = 600 meters.

- Train B is 150 meters longer than train A.

- Speed of train B is 10 m/s and it crosses a pole in 40 seconds.

- Therefore, the length of train B = 10 m/s × 40 s = 400 meters.

- Using the length of train B, we can say:

- Length of train A = 400 meters - 150 meters = 250 meters.

- Substitute back to find platform's length:

- Total distance (600 meters) = length of train A (250 meters) + length of platform.

- Length of platform = 600 meters - 250 meters = 350 meters.

- Correct answer: Option 4, 350 meters