Harmonic unison and octave intervals are difficult to distinguish. It can be frustrating when taking a quiz, but it does reflect a real musical challenge.
The problem is that two notes an octave apart share the vast majority of the ‘overtones‘ from which they’re made. This means that if the two notes sound exactly together, you hear the combination almost as a single note with a slightly different timbre, rather than two distinct notes. This can make it very hard to distinguish it from a single note (unison).
People find this particularly hard with very high or very low notes (cases where the ear is generally able to hear less detail).
There are a couple of things you can do to help with these cases. First, you may find it really helps to use headphones – or if you already are, try a different pair. This can accentuate different frequencies in the notes and make it easier to tell when it’s unison or an octave.
The other option is to approach the harmonic case gradually, by using ascending and descending intervals but having the time between notes be very short. With our RelativePitch app you can set this in the app settings menu. This way the notes are separated enough to distinguish the two cases, but you mostly hear the two notes together, so your ear can tune into the harmon