**Correct option (c) 36 days **

**Explanation: **

According to question, Efficiency A : B

2 : 1

If B does 1 unit of work A will do 2 units of work. So in a day,

they will complete 3 units of work together.

Total work = 12 days × 3 units/day = 36 units

B can complete the whole work alone in

⇒ 36/1 = 36 days