Explanation: Waveguide theory is based on principles that govern the behavior of electromagnetic waves at very high frequencies. **Skin effect** is fundamental because at microwave frequencies, current flows almost exclusively on the surface of a conductor. In a waveguide, this means the inner surfaces effectively become the "conductors" that confine and guide the electromagnetic fields, preventing radiation and ensuring efficient propagation. The **use of ¼ wave stubs** highlights the importance of resonant structures and transmission line principles in microwave engineering. While not literally stubs inserted into a waveguide for basic operation, the concept of using physical dimensions (like quarter-wavelengths) to achieve impedance matching, create resonant cavities, or filter frequencies is essential for designing and understanding waveguide components and their behavior, including cutoff frequencies and propagation modes. **Ohm's Law** (A) is a basic circuit theory principle and does not describe the complex electromagnetic wave propagation in waveguides. **High standing waves** (B) are generally undesirable in transmission lines and waveguides, indicating reflections and power loss, rather than a foundational principle of waveguide operation itself.