Explanation: A synchronizer is essential for maintaining precise timing or phase relationships, especially critical in complex signal generation. The **modulator** is where information is imposed onto a carrier wave. For many advanced modulation schemes, such as digital modulation (e.g., PSK, QAM), or even analog schemes like FM stereo multiplexing or television video signals, accurate timing and phase synchronization are vital to correctly encode the information. The synchronizer ensures the modulator operates with the necessary stability and alignment for the modulated signal to be intelligible and decodable by a receiver. A mixer (A) combines frequencies but doesn't primarily depend on a synchronizer for its fundamental operation. A receiver (B) *uses* synchronization for demodulation but is not primarily *affected* by a synchronizer in the context of signal generation. An I.F. amplifier (D) simply amplifies a signal and has no direct synchronization function.