What do "RST" signal reports describe?

The RST system is a standard method used by amateur radio operators to exchange information about the quality of the signal they are receiving from each other. It provides a quick, standardized way to report how well a signal is being copied by the receiving station. The other options refer to completely different aspects of radio physics or equipment. For example, ionospheric conditions and sunspot activity relate to signal propagation, not the direct characteristics of the received audio or continuous wave (CW) signal. Transmitter power is what the sender outputs, not what the receiver ultimately hears.