What type of signaling to a repeater uses two simultaneous audio tones?

DTMF stands for Dual-Tone Multi-Frequency. As the name suggests, it combines two specific simultaneous audio tones (one high group tone and one low group tone) to represent a single number or symbol on a keypad. This is the same technology used in traditional touch-tone telephones and is widely used in amateur radio to send control commands to repeaters or to dial into linked networks like EchoLink or IRLP. The other options do not fit this description. CTCSS uses only a single sub-audible tone to open a repeater's squelch. GMRS is an entirely different radio service, and D-STAR is a digital voice and data protocol, not an analog dual-tone signaling method.