The correct answer is A. Positive references to the audience's sense of right versus wrong.
B is incorrect because this would be the appeal to emotion, which is pathos, rather than ethos. C is incorrect because this is the appeal to logic, which is logos, rather than ethos. D is incorrect because this won't appeal to the audience much.
This leaves us with A, because ethics has to do with morals, with your understanding of what is right and wrong.