Explanation:

Let,   B1 : B2 : B3 = 3x : 4x : 5x

again  B1 : B2 : B3 = 5y : 4y : 3y

Since there is increase in no.of oranges in first two baskets only,

it means the no. of oranges remains constant in the third basket

Therefore,   5x = 3y

Hence    3x : 4x : 5x   =>   9y/5:12y/5:15y/5

 = 9y:12y:15y

and 5y : 4y : 3y   =>   25x : 20x : 15x

Therfore, increment in first basket = 16

Increment in second basket = 8

Thus, required ratio = 16/8 = 2:1

Hence, option 2 is the correct answer.