Which of the following is NOT a condition that affects radio wave propagation?

Resonance does not directly affect radio wave propagation in the same way that the other listed options do.

Refraction, reflection, and absorption are all physical phenomena that influence how radio waves travel through different media. Refraction refers to the bending of waves as they pass through different layers of the atmosphere, which can alter their path and speed. Reflection occurs when waves bounce off surfaces, such as the ground or buildings, which can impact their direction and strength. Absorption happens when waves are absorbed by materials, reducing their intensity as they pass through obstacles or propagating in different environments.

In contrast, resonance is a concept that applies more to systems vibrating at their natural frequencies rather than to how radio waves propagate through and interact with their environment. Therefore, it is less relevant in the context of conditions affecting the travel of radio waves, making it the correct choice as the exception.