Explanation: A **frequency discriminator** is specifically designed to detect FM-phone signals. Its fundamental operation involves converting variations in the incoming signal's frequency into proportional variations in amplitude. Since FM (Frequency Modulation) encodes audio information as changes in the carrier frequency, the discriminator effectively "demodulates" the signal, recovering the original audio waveform. A **balanced modulator** (A) is used to generate amplitude-modulated signals, specifically those with a suppressed carrier, such as Double Sideband Suppressed Carrier (DSB-SC) or as a stage in Single Sideband (SSB) generation—it does not detect FM. A **product detector** (C) is primarily used for detecting CW (Morse code) and SSB signals by mixing them with a local beat frequency oscillator (BFO) to restore the audio, which is a different principle than FM detection. A **phase splitter** (D) creates two output signals that are 180 degrees out of phase from a single input, commonly used in amplifier stages, but it is not a demodulator.