For a parallel plate capacitor filled with two dielectric materials, the effective dielectric constant is given by the formula \( \frac{K_1 + K_2}{K_1 K_2} \), considering the properties of the materials.
Step 2: Conclusion.
The correct ratio is \( \frac{K_1 + K_2}{K_1 K_2} \), corresponding to option (c).