Explanation: GSM (Global System for Mobile Communications) primarily uses a voice CODEC based on Regular-Pulse Excited-Linear Predictive Coding (RPE-LPC). This technique efficiently encodes speech by modeling the human vocal tract through linear prediction. The "excitation" signal, which mimics the sound source (like vocal cord vibrations), is represented by a series of regularly spaced pulses. This method achieves effective speech compression while maintaining acceptable quality, which was crucial for optimizing bandwidth in early digital mobile radio systems. Options B, C, and D are incorrect. "Code-Excited Linear Predictive (CLEP)" and "Linear Excited Code (LEC)" are not standard, recognized CODEC types. While "Multi-Pulse Excited (MPE)" codecs do exist and use a different excitation model, RPE is the specific and foundational excitation method used by GSM for its primary voice codec.