d
Second overtone of an open organ pipe

\(\text { (Third harmonic) }=3 \times v_{0}^{\prime}=3 \times \frac{v}{2 L^{\prime}}\)

First overtone of a closed organ pipe

\(\text { (Third harmonic) }=3 \times v_{0}=3 \times \frac{v}{4 L}\)

According to question,

\(3 v_{0}^{\prime}=3 v_{0} \Rightarrow 3 \times \frac{v}{2 L^{\prime}}=3 \times \frac{v}{4 L}\)

\(\therefore \quad L^{\prime}=2 L\)