Here is just my opinion,
"For which of the following conclusions does the passage offer the least support?"
First of all, you should ask, "who wrote the passage?" --- The author.
"whose conlusions does the passage support (or least support)?" --- The author's conclusions.
However, if the question was worded another way, then you were right.
"Which of the following conclusions offer the least support?". There is no 1st person here and it could be anyone's conclusions.
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"The passage suggests that its author would probably disagree with which of the following statements?"
I would have to look into this passage to know what you are talking 'bout. The correct answer shouldn't have to explicitly be negated in the passage. You just pick the "best" answer out of four but keep in mind that the answer must be responsive to the passage in order for the author to disagree.
