Explanation:

- Option 1: 20 days

- Calculate total work as a fraction for each: A = 1/20, B = 1/35, C = 1/60.

- Total work per day by A, B, and C together: $\frac{1}{20}+\frac{1}{35}+\frac{1}{60}$.

- Simplify this to find combined work rate and determine total days needed by subtracting B and C's last days of work.

- Option 2: 15 days

- Process similarly as above.

- Ensure work completion considers B leaving 8 days earlier and C 12 days earlier.

- Check if total days align with Option 2's claim.

- Option 3: 10 days

- Complete calculation in the same manner.

- Verify if it fits conditions of B and C leaving earlier.

- Too few days considering rates.

- Option 4: 12 days

- Repeat calculations.

- Shorter than realistically possible due to A's slower work rate.

The Correct Answer is Option: 2 - 15 days

Calculations confirm total days are 15 when accounting for B and C leaving earlier.