Now as one pipe fills the cistern and other empties it, 

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.

$\frac{5\times 8}{8-5}=\frac{40}{3}$

Since $\frac{40}{3}$^rd part is emptied in 1 hour.

And $\frac{3}{4}$th part will be empties in

$\frac{40}{3}\times \frac{3}{4}=10 \mathrm{hours}$

Hence option B is correct.