The correct answer is B: Frequency synthesis and FM demodulation. A phase-locked loop (PLL) can perform frequency synthesis (generating multiple frequencies from a single reference) and FM demodulation (extracting the audio from an FM signal). These are two of the most common PLL applications. In frequency synthesis, the PLL multiplies or divides the reference frequency to generate different output frequencies. This is how modern transceivers generate all their operating frequencies from a single crystal reference. In FM demodulation, the PLL locks to the incoming FM signal, and the control voltage that adjusts the VCO to maintain lock is proportional to the audio modulation. PLLs are versatile circuits that can perform both functions, making them essential in modern radio equipment.