The correct answer is C: Receive audio, transmit audio, and transmitter keying. In a computer-radio interface for digital mode operation, the key signals are receive audio (from radio to computer), transmit audio (from computer to radio), and transmitter keying (PTT control from computer to radio). Digital modes require the computer to receive audio from the radio (to decode received signals), send audio to the radio (to transmit encoded signals), and control when the radio transmits (PTT/keying). Some interfaces also provide CAT (Computer Aided Transceiver) control for frequency and mode control, but the three essential signals are audio in, audio out, and PTT. These three connections allow full digital mode operation.