What does 'Ottava, 8va' refer to in musical notation?

'Ottava, 8va' in musical notation indicates that a passage of music should be played one octave higher than written. This marking is used to simplify the notation for higher pitches, allowing the composer to avoid excessive use of ledger lines above the staff.

When the term '8va' appears above a staff, it instructs the performer to take all the notes indicated in that section and transpose them up by twelve semitones — effectively moving each note one octave higher. This is essential in certain musical contexts where the original pitch might become difficult to read or play due to being too high, making the notation more accessible.

The other options do not relate to the concept of octave transposition: the sixth note, quarter note, and tempo are all distinct aspects of music that do not connect with the idea of playing notes an octave higher. Thus, recognizing 'Ottava, 8va' as referring to the interval of an octave is crucial for understanding how to interpret various instructions in musical scores.