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's settings to be configured exactly right. If your transceiver's frequency offset is set incorrectly (either transmitting in the wrong direction or with the wrong frequency split), you will be transmitting on a frequency the repeater is simply not listening to. Additionally, most modern repeaters require a "key" to open their squelch to prevent local interference from activating them. If you have the wrong CTCSS tone (Continuous Tone-Coded Squelch System) or the wrong DCS code (Digital Coded Squelch) programmed, the repeater will ignore your transmission entirely, even if you are on the correct frequency. Because all of these scenarios will cause a failure to access the repeater, all choices are correct.