Correct option is B
Given
Time taken by (A + B) = 2 days
Time taken by B alone = 4 days
Target = Double the work by A alone
Formula Used
Solution
Let A's one day work be
Time taken by A alone to complete the original work = 4 days
Time taken by A to complete double the work = 4 × 2 = 8 days
Final Answer
So the correct answer is (b)