What type of signaling uses pairs of audio tones?

DTMF stands for Dual-Tone Multi-Frequency. As the name implies, every time you press a button on a radio keypad, the transmitter sends a unique pair of audio frequencies simultaneously. This is the exact same system used by traditional touch-tone landline telephones to dial numbers or send commands. The other options are incorrect because they refer to entirely different technologies. CTCSS (Continuous Tone-Coded Squelch System) uses a single continuous, subaudible tone, not a pair of audible tones. D-STAR is a digital voice protocol, and GPRS is a cellular data service.