Because different manufacturers use different labels for octaves, it’s best to determine which MIDI Note numbers your external keyboard sends out. As stated, the AR responds to MIDI Notes 12-59 chromatically.
Whether the pitch you hear is a C depends also on the sound settings: the analog engines need careful tuning and the samples used need to be of a known tuning in order to ensure that you hear a C. There is a lot of discussion on the forum about tuning:
The machines need to be carefully tuned; many are designed for percussion sounds, not as melodic sounds. They may be difficult to keep in tune across the full 4-octave range.