What type of signaling uses pairs of audio tones?

The correct answer is DTMF, which stands for Dual-Tone Multi-Frequency signaling. This system utilizes pairs of audio tones to convey information, especially in telephone communication. Each key on a touch-tone keypad generates two specific frequencies that are transmitted simultaneously, one low frequency and one high frequency. When a key is pressed, the unique combination of these two frequencies represents a specific digit or command.

This unique method of signaling allows for precise and efficient communication by ensuring that each key can be distinctly identified by the receiving device. DTMF is widely used in modern telecommunication for prompts in automated systems, voicemail, and navigation through phone menus.

The other options are relevant technologies but operate differently. CTCSS (Continuous Tone-Coded Squelch System) employs specific sub-audible tones for ensuring selective calling and reducing interference, rather than pairs of tones to represent multiple digits. GPRS (General Packet Radio Service) is a mobile data standard for cellular networks, while D-STAR (Digital Smart Technologies for Amateur Radio) is a digital voice and data protocol used in amateur radio, focusing on digital data transmission rather than tone signaling.