Explanation:

- A completes the entire work in 12 days. So, A's one-day work is 1/12 of the work.

- B completes the entire work in 20 days. So, B's one-day work is 1/20 of the work.

- When A and B work together for 5 days, their combined one-day work is (1/12 + 1/20).

- Calculating the combined work:

$$

\frac{1}{12} + \frac{1}{20} = \frac{5 + 3}{60} = \frac{8}{60} = \frac{2}{15}

$$

- In 5 days, they complete:

$$

5 \times \frac{2}{15} = \frac{10}{15} = \frac{2}{3}

$$

- So, 1/3 of the work remains.

- C completes this remaining 1/3 of work in 3 days.

- Therefore, 3 days is 1/3 of the time C would take to do the whole work alone.

- So, C can complete the work alone in:

$$

3 \times 3 = 9 \text{ days}

$$

- Option 2: 9 days is the correct answer.