In Psychological Research, What Is a Confederate

In psychological research, what is a confederate?

A) an experimenter who has prior knowledge that conditions unaccounted for in the study's design will affect the study's outcome
B) any variable that is held constant throughout the study and measured at the end
C) someone who pretends to be a participant in a research experiment but actually works for the experimenter
D) an assigned group of participants who experience all aspects of the study except the independent variable