The characteristic that does not signify the Plainchant is:
Accompanied by a pipe organ
Step-by-step explanation:
The Plainchant is a form of Gregorian chants and was a form of medieval church music across Europe.
It was a body of chants that was widely used in the liturgies across the Western Church. The Plainchant uses monophonic melody, which means there is one melodic line that is unaccompanied.
The unaccompanied line means that there is no need of any sort of instrumentation for the music as it only needs human voice for its function so there is no need for a pipe organ in this form.