According to the formula, If Two pipes A and B can fill the tank in x and y min respectively. If both pipes are opened together, then the time after which pipe B should be closed so that the tank is full in t min is $\left\{ \mathrm{y} \left[1-\frac{\mathrm{t}}{\mathrm{x}}\right] \right\}$ hours.

So, $\left\{ 4 \left[1-\frac{6}{12}\right] \right\}=4\times \frac{6}{12}=2 \mathrm{hours}$

Hence, the tank will be filled in 2 hours both the pipes are opened simultaneously.

Hence option C is correct.