Final answer:
The word that best fits the definition of music with a lively, rhythmic sound is 'rhythm', which is the pattern of sounds and silences in music.
Step-by-step explanation:
The rhythm is the aspect of music with a lively, rhythmic sound. Rhythm can be described as the "heartbeat" of music, where sounds and silences are organized in time to create patterns. In contrast, a beat is the basic unit of time in music, melody refers to a sequence of notes that are perceived as a single entity, and harmony refers to the combination of different musical notes played or sung simultaneously. Tempo, on the other hand, is the speed of the underlying beat. Therefore, out of the options given - melody, harmony, rhythm, and tempo - rhythm best fits the definition of music with a lively, rhythmic sound.