Explanation:

To calculate how long A, B, and C take to finish the job together:

- A completes the job in 10 days, so A has a work rate of 1/10 of the job per day.

- B completes the job in 15 days, so B has a work rate of 1/15 of the job per day.

- C completes the job in 6 days, so C has a work rate of 1/6 of the job per day.

- Add their work rates to find the combined work rate:

$$

\frac{1}{10} + \frac{1}{15} + \frac{1}{6} = \frac{3}{30} + \frac{2}{30} + \frac{5}{30} = \frac{10}{30} = \frac{1}{3}

$$

- The combined work rate of A, B, and C is 1/3 of the job per day.

- They can finish the job in 3 days, as 1/(1/3) = 3 days.

- Correct Answer: Option 2 - 3 days