Which of the following could be the reason you are unable to access a repeater whose output you can hear?

Accessing a repeater requires your radio to transmit on the repeater's specific listening frequency and use the correct access tones. If your transceiver offset is improper (for example, set to a positive offset instead of a negative one, or set to simplex), you will transmit on a frequency the repeater is simply not monitoring. Furthermore, repeaters frequently use Continuous Tone-Coded Squelch System (CTCSS) or Digital Coded Squelch (DCS) to prevent interference from other signals or noise. If you have not programmed the exact sub-audible tone or digital code required by the repeater, its receiver will remain squelched and ignore your transmission, even if you are on the correct frequency. Therefore, all of these issues are valid reasons for failing to access a repeater.