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

$\frac{42 \mathrm{parts}}{6 \mathrm{hours}}=\frac{7 \mathrm{parts}}{1 \mathrm{hour}}$

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

$=\frac{14 \mathrm{parts}}{2 \mathrm{hour}}$

Together they complete 14 parts in 2 hours

And the remaining 28 part will be filled by A and B in 7 hours

$\frac{28 \mathrm{parts}}{7 \mathrm{hours}}=\frac{4 \mathrm{parts}}{1 \mathrm{hour}}$

A and B together can fill 4 parts in 1 hour

From this we could find C’s filling parts per hour
(A+B+C)-( A+B)=C

7-4= 3 parts

Total 42 parts of the tank will be filled by C alone

$\frac{42}{3}=14 \mathrm{hours}$

Hence option D is correct.