Correct option is A
Total work = 12 units
Efficiency of A+B = 12/12 = 1 unit
Efficiency of A = 2/3 units
Efficiency of B = 1/3 units
Efficiency of A+B+C = 12/6 = 2 units
Efficiency of C = 1unit
Time taken by C = 12
R + 8 = 12
R = 4