The speed of all radio signals is characterized as:

The speed of all radio signals is characterized as always at the speed of light, which is approximately 299,792 kilometers per second (or about 186,282 miles per second) in a vacuum. This speed is a fundamental constant of nature, as described by Einstein's theory of relativity, and it applies to all forms of electromagnetic radiation, including radio waves.

While radio signals can propagate through different media (like air, water, or optical fibers), and their speed may vary slightly in those media compared to their speed in a vacuum, they are fundamentally bound by the speed of light. Thus, regardless of environmental factors, when considering radio transmission, it is accurate to state that radio signals travel at the speed of light in a vacuum.

The other options introduce concepts that might mislead understanding. The notion of radio signals being constant but variable with distance falsely suggests that the speed changes with distance, which it does not; it remains constant. Saying the speed is greater than the speed of sound is also true but does not fully capture the fundamental nature of radio signals' speed. Finally, while it is true that the speed of signals can be affected by the medium through which they travel, this does not negate the fact that in a vacuum,