The correct answer is answer choice C, 'Does my writing communicate a clear and well-supported idea?'. It's important to ask each of the other questions as well, however, answer choice A is better when proofreading your essay, answer choice B is better for evaluating the validity of the information in your essay, and answer choice D is better when planning and essay and deciding how to go about writing it. Asking whether or not your essay is clear and well-supported is vital when determining the clarity of it, because an essay with clarity should be clear and easy to understand for the reader.
Hope this helps!