Whether more people like chocolate vs. vanilla is not at issue. The issue is what explains why people like low-fat vs. full-fat chocolate equally, whereas they like full-fat vanilla more than low-fat vanilla.

Whether the subjects in previous tests knew about the fat differences has no clear impact on what explains why people like low-fat and full-fat chocolate ice cream equally.

This suggests a more complex flavor will be more liked than a less complex flavor. But we’re concerned with comparisons between high-fat and low-fat within the same flavor. Why do people like low-fat vanilla less than high-fat vanilla, but like both versions of chocolate equally?

If vanilla were just as complex as chocolate, the author’s hypothesis about complexity masking taste wouldn’t make sense, because people dislike low-fat vanilla compared to high-fat. (D) defends the argument by confirming that vanilla is less complex than chocolate.

People’s awareness of complexities of flavors has no clear impact. They may be aware, but we have no reason to think this relates to complexity of flavor helping to mask taste differences between high-fat and low-fat ice creams.