A is twice as efficient as B. Together, they can complete a task in 12 days. How many days will A alone take to complete the task?

Correct option is B

Given
Efficiency of A = $$2 \times$$​ Efficiency of B
Time taken by A and B working together = 12 days

Formula Used
Total Work = Total Efficiency $$\times$$ Number of Days
Time taken by A = $$\frac{Total \ Work }{Efficiency \ of \ A}$$​​

Solution
Let the efficiency (work units done per day) of B be 1 unit/day.
Then, the efficiency of A is 2 units/day.
Combined efficiency of A and B = 2 + 1 = 3 units/day.
Total Work = Combined Efficiency $$\times$$​ Days worked together
Total Work = $$3 \times 12$$​ = 36 units.
Time taken by A alone = $$\frac{36}{2}$$​​
Time taken by A = 18 days.

Final Answer
So the correct answer is (b)