Giving a counterargument allows the essay/writing to contain more perspectives that strengthen the claims as more perspectives allow the reader/viewer to see an overall "picture" of the argument. Also, being able to "debunk" the counterargument as false or not strong will help in strengthening your own essay/argument.
Including only your side of the argument leaves the reader/viewer no perspective of the whole topic.
The answer is 'colluded', because it means that they schemed to do it secretly. The other options aren't possible here, and most of them are adjectives, and you need a verb.