Answer is (3)
For closed organ pipe, third harmonic will be $\frac{3 v}{4 l}$
For open organ pipe, fundamental frequency will be$\frac{v}{2 l^{\prime}}$
Given,
$\frac{3 v}{4 l}=\frac{v}{2 l^{\prime}}$
$\Rightarrow \quad l^{\prime}=\frac{4 l}{3 \times 2}=\frac{2 l}{3}$
$=\frac{2 \times 20}{3}=13.33 \mathrm{~cm}$