Step 1: Count overtones:
The first overtone of a closed pipe is the 3rd harmonic. The fifth overtone is the harmonic $2 \times 5 + 1 = 11$. For an open pipe the first overtone is the 2nd harmonic, so the fifth is the 6th.
Step 2: Equate wavelengths:
$\lambda_o = \frac{2L_o}{6} = \frac{L_o}{3}$ and $\lambda_c = \frac{4L_c}{11}$. Equal frequency means equal wavelength, so $\frac{L_o}{3} = \frac{4L_c}{11}$, giving $\frac{L_o}{L_c} = \frac{12}{11}$.
Final Answer:
The ratio is $12:11$, option (D).
\[ \boxed{12:11} \]