Explanation:

We know that, when the trains are moving in the same direction,
Relative speed= Difference between the speed of train

and total distance for crossing each other = Sum of the length of train

Therefore, Relative speed = 72 km/hr - 54 km /hr = 18 km/hr
Relative speed in m/sec = 18 km/hr x 5/18 = 5 m/sec

and Total distance = 200m + 300m = 500m

and Total time taken to cross each other = Total distance in meter/ Relative speed in m/sec

= 500 m / (5 m/sec) = 100 sec

Hence, option 2 is the correct answer.