Explanation:

The train can cover a distance equal to its length in 15 seconds.

The train can cover (a distance equal to its length + 100 metre) in 25 seconds.
Therefore, it can be concluded that the train can travel 100 metre in (25−15)=10 seconds

and therefore its speed is 100/10=10 m/s
Hence, length of the train =10×15=150 metre

Hence, option 1 is the correct answer.