Explanation:

- The train's speed is 108 km/hr, which converts to 30 m/sec.

- The train crosses the platform in 40 seconds.

- Total distance covered by train to clear the platform = train length + platform length.

- The platform length = train's speed x time - train length = (30 m/sec * 40 secs) - 720 meters = 480 meters.

- The man crosses the same platform in 120 seconds.

- Man's speed = platform length / time = 480 meters / 120 seconds.

- This simplifies to a speed of 4 m/sec.

- Therefore, the answer is Option 3: 4 m/sec .