According to the formula, if pipe A can comletely fill a tank in x hours and pipe B can empty the same tank in y hours, then the time taken to fill the tank when both the pipes are opened simultaneously is $\frac{\mathrm{xy}}{\mathrm{x}-\mathrm{y}}$hours.

so, $\frac{4\times 8}{8-4}=\frac{32}{4}=8 \mathrm{hours}$

The tank will be filled in 8 hours when both the pipes are opened simultaneously.

Hence option D is correct