Which of the following is used to convert a signal from one frequency to another?

A mixer is an RF circuit that takes two input signals and combines them to produce new output signals, specifically at the sum and difference of the original frequencies. This process, known as heterodyning, is the standard method used in radios to shift a signal from one frequency to another. The other options serve entirely different purposes. An amplifier increases the strength or power of a signal without changing its frequency. A phase splitter divides a single signal into multiple paths with different phases, and an inverter simply flips the polarity of the signal.