Which of the following is an appropriate power supply rating for a typical 50-watt output mobile FM transceiver?

Answer: D. 13.8 volts at 12 amperes

Mobile radios are built to run from a car's electrical system, which is about 13.8 volts while the engine is running, so the answer is 13.8 volts at 12 amperes. A 50-watt transmitter is not 100% efficient, so it draws much more than 50 watts of input power. 13.8 V x 4 A is only about 55 W, which leaves no headroom, while 13.8 V x 12 A is about 166 W, which is plenty.

Tip: Mobile rigs: 13.8 V. Pick a supply with current to spare.