Explanation: In pulse-code modulation (PCM) combined with time-division multiplex (TDM), an analog signal is converted into a digital stream of pulses, and multiple such digital streams are interleaved in specific time slots on a single channel. For the receiver to correctly reconstruct the original individual signals, it must know exactly when each time slot begins and ends and which pulses belong to which signal. This requires the receiver's internal clock to be perfectly aligned, or synchronized, with the transmitter's clock. Without precise synchronization, the receiver would misinterpret the data stream, scrambling the information from different channels or failing to decode it altogether. * **B) Frequency separation:** This is characteristic of Frequency-Division Multiplexing (FDM), where different signals occupy distinct frequency bands, not TDM. * **C) Overmodulation and undermodulation:** These terms primarily apply to analog amplitude modulation (AM) and refer to the depth of modulation. PCM is a digital process and these specific concepts are not the defining factor for its operation. * **D) Slight variations in power supply voltage:** While stable power is always beneficial, digital circuits typically have power regulation to handle minor variations, and such fluctuations are not the primary critical factor for the fundamental operation and decoding of TDM/PCM compared to precise timing.