This is actually a recurring answer choice in assumption questions. It is virtually never correct when the answer choice states that something is the "most important" factor or consideration in an argument. We simply cannot know whether that is necessarily true or not.
Keep in mind that if you negate this, it would become "is not the most important factor". This difference doesn't really affect the argument. For example something can be highly important but be the second most important factor.
I hope this helps. Please let me know if you have any other questions.
