This is how I understand it. Since it is a sufficient assumption question, we want to pick the answer that will enable the conclusion to be drawn 100%. So, by making bitterness the only factor relevant, it gets rid of the potential role of 'also' that may account for other factors, such as saltiness. For sufficient assumptions, the question isn't how the passage makes bitterness the ONLY factor relevant. The question is what answer choice allows the conclusion to be properly drawn, which is D).

Are there any indicators to know whether an SA question is rule-application or find/complete the application?

In PT's, should we always be thinking of alternative hypotheses before looking at the answers? I feel like this can unnecessarily take up time.