We can see here that the voice-leading error from the list is: parallels.
Parallel voice leading, also known as "parallel fifths" or "parallel octaves," refers to a specific error or phenomenon in music composition, particularly in harmony or counterpoint.
It occurs when two voices (melodic lines or parts) move in the same direction by perfect intervals, such as a fifth or an octave, while maintaining the same interval between them. In other words, if two voices start at a perfect interval (like a fifth) and move to another perfect interval (like another fifth) in parallel motion, it creates a consecutive and similar harmonic progression that might sound bland or lacking in harmonic variety.