They knew by his sophomoric remarks that he was still naive in the field.

In most contexts, "sophomoric" conveys that something is juvenile or lacking in maturity; this makes Option C the correct choice. The sentence implies that the person’s comments (remarks) revealed their lack of maturity or depth, making it clear they were still inexperienced or naive in that area. Option D may seem like a close contender, but ignorance is not inherently the same as immaturity (though the former can sometimes contribute to the latter). Option A mentions "unpalatable", which means unpleasant or disagreeable; this does not fit the given text. Similarly, we can eliminate Option B - "casual" could imply a more informal remark; however, this does not tie in with the judgement of the person being naive (according to the speaker).

Hence, Option C is the best choice.