d
\(\frac{\lambda}{4}=\mathrm{L}_1 \quad 2\left(\frac{\lambda}{2}\right)=\lambda\)

\(\mathrm{v}=\mathrm{f} \lambda \quad \mathrm{f}_2=\frac{2 \mathrm{v}}{2 \mathrm{~L}_2}\)

\(\mathrm{v}=\mathrm{f}_1\left(4 \mathrm{~L}_1\right) \quad \mathrm{f}_2=\frac{\mathrm{v}}{\mathrm{L}_2}\)

\({v}=\mathrm{f}_1\left(4 \mathrm{~L}_1\right)\)

\({f}_2=\frac{\mathrm{v}}{\mathrm{L}_2}\)

\({f}_1=\frac{\mathrm{v}}{4 \mathrm{~L}_1}\)

\({f}_1=\mathrm{f}_2\)

\(\frac{\mathrm{V}}{4 \mathrm{~L}_1}=\frac{\mathrm{V}}{\mathrm{L}_2}\)

\(\Rightarrow \mathrm{L}_2=4 \mathrm{~L}_1\)

\(60=4 \times \mathrm{L}_1\)

\({~L}_1=15 \mathrm{~cm} \)