The ratio is people to buses so that is the order in which the ratio should be set up in
People:Buses
150:6 (however you are not done, this can be simplified)
Try by dividing 6 from either sides.
In that case you will get 25:1
BTW, don't be fooled by the other options
The answer to F is wrong because although ration is simplied, its in the wrong order
The answer H is wrong because its in the wrong order and is not simplified
Though the answer to I seems right because of the order, its not simplified and lacks the : symbol