If I understand your question correctly, we refer to the writer of the stimulus as the "author" unless it specifies otherwise. The unnamed person making the argument is the author. So if there is a flaw in the argument, we will say that the author has made a reasoning error.
Sometimes it will say... Journalist: Critic: Scientist: Editor:
In those cases, a question stem might say, "A flaw in the argument is that the editor assumes..." but these are all referring to the person writing the stimulus.
