Final answer:
The term to describe how a tone is held when two notes are played together is 'sustained.' 'Harmonious' and 'resonant' also align with the reference material provided, describing relationships of pitch and resonance respectively.
Step-by-step explanation:
The question asks for the correct term to describe how a tone is held when two notes are played together. The appropriate term to fill in the blank is 'sustained,' since the question implies that the notes are held for a duration rather than describing their harmonic relationship. However, if the intention was to describe notes played together, the more specific terms related to sound and music theory, as provided in the contextual clues, are harmonious, dissonant, and resonant.
Harmonics are all multiples of the fundamental frequency, and a resonance will occur when the frequency formed is a harmonic and moreover, overtone as indicated in the reference material provided. Therefore, among the provided options (harmonious, dissonant, resonant, sustained), harmonious and resonant seem to be the most contextually appropriate. However, if the question exclusively refers to the duration the notes are held, 'sustained' is correct.