So, first you need to find out how much is for one pair of jeans. To find this out, you need to divide the cost by quantity.
Old Navy: 24÷3=8 So, $8 for one pair or jeans
Wet Seal: 16÷2= 8 So, $8 for one pair of jeans
Marshalls: 39÷4= 9.75
So, your answer is Old Navy or wet seal