A train crosses a bridge in 30 seconds at the speed of 72 km/h. If the length of train is 250 m, then what is the length of the bridge?

Converting the speed of the train from km/hr into m/s, we get $$72\times\ \frac{5}{18}=20$$ m/s

To completely cross the bridge, the train end would have entered from one end, and the end of the train would have to come out, which, we are given, makes 30 seconds. 
The total distance covered by the engine of the train int his distance would be the train's length and bridge's length. 

$$20\times\ 30=250+$$ Bridge length
Bridge length $$=600-250=350$$ m

Therefore, option B is the correct answer.