You simply need to find the price of one orange at each store, and multiply that by 30.
Store A: 5/3= around $1.67= 1.67*30= $50.10
Store B: 3/2= $1.50= 1.5*30= $45.00
Store C: 9/5= $1.80= 1.8*30= $54.00
Store D: $1.25= 1.25*30= $37.50
As you can see above, at Store D, 30 oranges would only cost $37.50, making it the cheapest store to buy oranges.
I hope this helps :)