D.
The ratio between A and B is A:3B. For B to be the limiting reagent, there has to be less than 3B for every A.
For A:4B, there are 4B for every A.
For A:5B, there are 5B for every A.
For 2A:7B, there are 3.5B for every A.
For A:B, there is only 1B for every A, which makes B the limiting reagent because no matter how much A there is, the reaction will be limited by the amount of B which is too little.
