Answer : Option A

Explanation :

Note: Assume the total work = LCM of the given days

Take the LCM of days = LCM of (18, 27, and 36) = 108
Let the total work = 108

A's one day work = 108/18 = 6 units
B's one day work = 108/27= 4 units
C's one day work = 108/36= 3 units

ATQ, A works only for 8 days.
So, A's total work done = 8 * 6 = 48
So, remaining work = 108-48 = 60 work done by (B and C).

If (B+C) work till last, they can finish the work in 60/7= 8.57 days.

But ATQ, B left the job 6 days before it is completed.
Assume that B is not left the work than B's 6-day work is also included in the total work.
So, B's 6 day work = 6 * 4 = 24 units

Now, the total remaining work = 60+24 = 84units.

(B+C)'s one day efficiency = 7

So, to complete the work it requires = 84/7 = 12 days.