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

The correct choice, a mixer, is an electronic device that combines two signals and shifts one to a different frequency, typically producing new frequencies at the output that are the sum and difference of the input frequencies. This function is essential in many communication systems, allowing for the upconversion or downconversion of a signal in applications like radio, television, and other forms of wireless communication.

Mixers enable operators to select specific frequencies for transmission or reception, facilitating the process of tuning to the desired channel while eliminating unwanted frequencies. This is particularly important in radio frequency (RF) communications where effective use of the spectrum is required.

Other options, while important in their own contexts, do not serve the same purpose. A phase splitter is designed to create two identical signals that are 180 degrees out of phase but does not inherently change signal frequency. An inverter primarily changes the polarity of a signal without altering its frequency, and an amplifier boosts the strength of a signal while keeping its frequency the same. Thus, the mixer stands out as the fundamental device for converting frequencies.