Let the number of filling pipes be ‘x’

So, the number of draining pipes will be (8 – x)

As per the question,
Each of the filling pipes can fill the tank in 8 hours.
So, in 1 hour each of the filling pipes fill $\frac{1}{8}$^th of the tank

Hence, in 1 hour x filling pipes will fill $\frac{\mathrm{n}}{8}$^th of the tank

Similarly,
each of the waste pipes can drain the full tank in 6 hours.
So in 1 hour each of the waste pipes will drain $\frac{1}{6}$^th of the tank

Hence, in 1 hour (8 – x) waste pipes will drain $\frac{8-x}{6}$^th of the tank

If for 1 hour, all filling and draining pipes are opened $\frac{1}{6}$^th of the tank gets drained

(water filled in 1 hour – water drained 1 hour) = $\frac{1}{6}$^th of the tank

Therefore,

$\frac{x}{8}-\frac{8-x}{6}=-\frac{1}{6}$

(As the tank is getting drained we have negative sign)

Upon cross multiplication and solving the equations we get

$$\begin{gathered} \Rightarrow 14\mathrm{n}-64=-8 \\ \Rightarrow 14\mathrm{n}=64-8 \\ \Rightarrow 14\mathrm{n}=56 \\ \Rightarrow \mathrm{n}=4 \end{gathered}$$

 Hence option B is correct.