An open exam question / essay topic gives the writer the illusion of free choice. You can say anything you like as long as you support your opinion and state everything coherently, logically, and grammatically.

Of course when you submit the answer to be graded / evaluated, you have no control over the inherent (and perhaps unconscious) biases of the grader(s)--and anyone who is honest knows that everyone harbors biases.

Even the supposedly objective measures of support, coherence, logic, and grammar impose constraints that the writer may not wish to follow for whatever reason (valid or not). Their main value is to promote obeisance to a certain framework of discourse and to confer on the evaluators the semblance of objectivity and the power to control the "conversation."

Based on what you've written here, I shouldn't be surprised that your Examination was scored with a D. Most likely, any risks you took in answering the question as you wanted to were punished appropriately by the invigilators.