The correct option is A 30 km/hr
Let the speed of the train be x km/hr.
Distance travelled = 210 km
Time taken to travel 210 km = 210x hours
When the speed is increased by 5 km/h, the new speed is (x+5)
Time taken to travel 210 km with the new speed is 210x+5 hours
According to the question,
210x−210x+5=1
⇒210(x+5)−210x=x(x+5)
⇒210x+1050−210x=x2+5x
⇒x2+5x−1050=0
⇒(x+35)(x−30)=0
⇒x=−35,30
The speed cannot be negative. Thus, the speed of the train is 30 km/hr