Final answer:
Harmony is the correct term for "simultaneous variations of a single line of music," referring to the way different notes come together to form chords and musical progressions.
Step-by-step explanation:
Harmony is defined as "simultaneous variations of a single line of music." This aspect of music involves the combination of different musical notes played or sung at the same time to produce a cohesive sound. In music, harmony refers to the construction of chords and the progression from one chord to the next. This can be seen in various forms of music where multiple notes or voices come together to create a richer, more complex sound than a single line of music such as a melody. With instruments like the flute and lyre, or even in vocal music, harmony combines with rhythm to create the full structure of a musical composition.
Rhythm, on the other hand, is the "heartbeat" of music, where words or notes fall into patterns creating the timing of the musical piece. Think of rhythm as the foundation that helps everything in music to move cohesively. In dancing, for instance, rhythm is used without harmony to convey emotions, character, and action through movement. It is the pattern of sounds and silences which create the tempo and feel of a piece of music.
Therefore, with references to the various elements and components of music mentioned, it is clear that the answer the student is looking for is A) Harmony.