A. False, a riff is a melodic phrase which is usually 2-4 bars long and repeats over a changing harmony.
B. False, a melody is a melodic phrase usually 3-6 bars long, which repeats over a changing harmony.
C. False, a riff is a melodic phrase usually 1 bar long which repeats over a changing melody.
Riffs are short melodic phrases that are repeated over a changing harmony. They are usually 2-4 bars long, although some may be 1 bar long. The melody of the riff may be the same throughout, or it may change slightly with each repetition. The harmony, however, will typically change with each repetition. Riffs are often used to create a catchy hook or to add texture to a song.