Explanation:

- The train crosses a pole in 12 seconds. This means the time taken to cover a distance equal to its own length is 12 seconds.

- The train takes 36 seconds to cross a bridge that is 170 meters long.

- This means the train's effective time to cover the length of the bridge, in addition to its own length, is 36 seconds.

To find the speed:

- The difference in time, 36 seconds - 12 seconds = 24 seconds, is the extra time taken to cross the bridge length.

- Since the bridge is 170 meters long, the train covers it in 24 seconds.

- Speed = Distance / Time = 170 meters / 24 seconds = 7.083 meters/second.

- Convert this to km/h: 7.083 m/s * 18/5 = 25.5 km/h.

- Correct Answer: Option 3 - 25.5 km/h