end, ${n_c} = \frac{{3v}}{{4{l_c}}}$.....$(i)$
Third harmonic or second overtone of organ pipe open at both end,
${n_0} = \frac{{3v}}{{2{l_0}}}$.....$(ii)$
given ${n_c} = {n_o}$
$ \Rightarrow \,\,\,\,\frac{{3v}}{{4{l_c}}} = \frac{{3{v_0}}}{{2{l_0}}}\,\,\,$
$\Rightarrow \,\,\frac{{{l_c}}}{{{l_o}}} = \frac{1}{2}.$

${z_1},{z_2}$ and ${z_3}$ as${z_1} = A\sin (kx - \omega \,t)$, ${z_2} = A\sin (kx + \omega \,t)$ and ${z_3} = A\sin (ky - \omega \,t)$.
Which of the following represents a standing wave