Which description best fits the term 'vocal monophony'?

Vocal monophony refers specifically to a single melodic line being sung by one or more voices without any harmonic accompaniment. This means that the focus is solely on the melody with no additional chords or harmonies that would add depth or complexity to the sound. A prime example of vocal monophony can be found in plainchant or Gregorian chant, where a solo singer or a choir might perform a single, unaccompanied melody, which allows for clarity and purity of the musical line.

The other descriptions involve multiple elements that do not align with the definition of vocal monophony. For instance, multiple vocal lines creating harmony would require polyphony, while instrumental accompaniment involves additional musical elements that also move away from the essence of monophony. Complex polyphonic textures indicate multiple intertwining melodies, which is the opposite of the singular focus of monophony. Consequently, the correct description of vocal monophony is the one that highlights a singular focus on melody without the presence of harmonic support or complexity.