I would go with the last sentence.
The question is whether those phrases are speaking of the same person :
<span>A. her friend and former room-mate
- this is the same person, if it was two people, it'd be "and her"
B. </span><span>the winner and still champion
-again, the same person, requires singular verb
C.</span><span>The chief and foremost expert
-again, the same person: requires a singular verb and actually has a singular verb, correct answer!
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