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