Dr. horton made it clear to his introductory psychology students that, if they didn't participate in his research, they would receive a failing grade in his class. what ethical principle has dr. horton violated?

He has violated the principle of freedom from coercion. When a subject is taking part in a survey or research, they must be doing so of their own will. Being tricked or coerced into participation is, with few exceptions, against many of the tenets of good Psych research: it will also likely give poor results that will have little practical effect in the larger world.