d
For open pipe, \(n=p \frac{v}{2 \ell}\) 

For closed pipe \(n^{\prime}=(2 p-1) \frac{v}{4 \ell}\)

\(\therefore \frac{\mathrm{n}}{\mathrm{n}^{\prime}}=\frac{2 \mathrm{p}}{(2 \mathrm{p}-1)}\)