A alone can complete a task in 3 days and B alone can complete the same task in 6 days. In how many days can A and B complete it together?

Let the total work be W

Number of days required for A alone to complete the task = 3

$$=$$>  Work done by A in 1 day = $$\frac{W}{3}$$

Number of days required for B alone to complete the task = 6

$$=$$> Work done by B in 1 day = $$\frac{W}{6}$$

Work done by A and B in 1 day = $$\frac{W}{3}+\frac{W}{6}=\frac{W}{2}$$

Number of days required for A and B together to complete the task = $$\frac{W}{\frac{W}{2}}=2$$

Hence, the correct answer is Option D