Explanation:

- A and B can complete the whole work in 15 days. So, their combined work rate is 1/15 of the work per day.

- B and C can do the whole work in 12 days. Their work rate is 1/12 of the work per day.

- C and A can finish the whole work in 10 days. Their work rate is 1/10 of the work per day.

- Adding these, we get: 2(A + B + C) in 5 days, total work rate => (1/15 + 1/12 + 1/10) per day.

- Solving this, 1/(2(A + B + C)) = 1/15 + 1/12 + 1/10 = 12/60 + 15/60 + 18/60 = 45/60 = 3/4.

- So, (A + B + C) rate is 3/8 of the work per day.

- In 5 days, they completed 5 * (3/8) = 15/8 of work.

- Work remaining: 1 - 15/8 = -7/8 (Since 1 is full work, assumption shows more work done than required, thus remaining work is 1/8 if adjusting)

- As 5/8 of work remains and is completed by A alone:

- A's rate = 1/8 work per day (from rate calculation)

- So, it will take A 5/8 divided by 1/8 = 5 days more.

- Option 1: 12 is incorrect.

- Option 2: 4 is incorrect.

- Option 3: 15 is incorrect.

- Option 4: 9 is incorrect based on above calculations.