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, $\frac{2\times 3}{3-2}=\frac{6}{1}=6 \mathrm{hour}$ 

Therefore, the cistern will be filled in 6 hours when both the pipes are opened simultaneously.

Hence option B is correct