Remember not to trust the official explanations too much: they are often imprecise and inconsistent. For that reason we wrote our own explanations in the GMAT Navigator (
https://www.manhattanprep.com/atlas/resource/58). Interestingly, the author of that explanation focuses on the words 'in importance' in answer B.
Dealing with the issue around the placement of 'only', I would say that the problem isn't that answer B is ambiguous, but that it suggests a different, illogical meaning. Consider the meaning of these sentences:
1. I'm second only to you in my Science exam.
2. I'm second to you only in my Science exam.Sentence 1 means that, in the Science exam, you got the highest score, and I was second. Sentence 2 means that you're better than me at Science, but I'm better than you at other subjects. Try to apply the same analysis to the answer choices in the problem.