Which of the following best describes the harmonic concept?

The concept of harmony in music specifically refers to the combination of different musical notes played or sung simultaneously, often forming chords. Harmony enriches a melody by providing depth and texture, creating a fuller sound that supports and complements the main melodic line. When discussing harmony, the focus is on how different notes interact with each other, resulting in consonant or dissonant sounds, which can evoke various emotions and enhance the overall musical experience.

The other options do not accurately encapsulate the concept of harmony. A single melody line pertains to melody rather than harmony, multiple voices singing in different tempos relates to vocal techniques or polyphony rather than harmony itself, and a sequential set of notes describes a melodic progression more than harmonic structure. Thus, the choice that best describes the harmonic concept is indeed chords or simultaneous notes.