Which of these vehicles must stop before crossing railroad tracks?

ANSWER:

Tank trucks marked with 'hazardous material placards' is the vehicle which must be stopped before crossing the railroad tracks.

EXPLANATION:

Basically, whenever the train is about to arrive or is about to pass, then all the vehicle must stop before crossing the rail road to prevent accidents.In this case, there are tanks with hazardous materials which may cause even more accident due to which the tank truck should be stopped before crossing the rail road.