The capacity of the tank can be obtained by finding LCM of 6, 2 and 7 which is 42.
Total parts of the tank is 42
A, B, C together can fill the tank in 6 hours
A,B and C can fill 7 parts in 1 hour
From the question A, B and C together are filling only for 2 hours
Together they complete 14 parts in 2 hours
And the remaining 28 part will be filled by A and B in 7 hours
A and B together can fill 4 parts in 1 hour
From this we could find C’s filling parts per hour
7-4= 3 parts
Total 42 parts of the tank will be filled by C alone
Hence option D is correct.