Which of the following causes failure of coaxial cables?

Coaxial cable is designed to protect the inner conductor and dielectric from the elements. However, if moisture enters the cable, it drastically alters the electrical characteristics of the dielectric insulator, causing a severe increase in signal loss and a high Standing Wave Ratio (SWR). Moisture contamination is the most common cause of coax failure in outdoor installations. The other options are incorrect: solder flux generally only affects the very ends at the connectors if applied improperly, and ordinary transmitter power fluctuations or high duty cycles will not damage a properly rated cable.