This argument holds that Downing did not act in a morally praiseworthy manner, despite being honest, as it was done out of self-interest. We are also given the condition that Morally Praiseworthy to be Honest -> Honest out of respect for morality.
Since the argument is trying to prove that Downing is not morally praiseworthy, we should predict something along the lines of his self-interest precludes him from being honest out of respect for morality. This would guarantee his not being praiseworthy.
The other thing I want to draw you attention to is that this is a strengthen with sufficient premise question. The negation of answer choices to destroy the argument is not a technique applicable to this question. This technique only applies to strengthen with necessary premise. Keep in mind, negating a sufficient condition never tells you anything, it is only the negation of a necessary condition which does tell you something. You should be focused on looking at the answer choices in the form they are given to you and determining whether they fully prove the argument or not.
If self-interest and doing something out of respect for morality are exclusive, then this is a solid argument.
I hope this helps. Please let me know if you have any questions.