Explanation:

- The train's speed is 108 km/hr, which needs to be converted to meters per second for easy calculation. This converts to 108 * (1000/3600) = 30 meters/second.

- The total distance to cover is the length of the train plus the length of the bridge: 300 + 900 = 1200 meters.

- Time to pass the bridge can be calculated using the formula: Time = Distance/Speed.

- Substituting the values, Time = 1200 m / 30 m/s = 40 seconds.

The correct answer is option:3 - 40 seconds.