You can say (b) in a case when the extra emphasis is needed. For example, — Kate and Sarah don't run fast. — Well, who does run fast?
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hirashinBut do you ever say (b) "Who does run fast?"It would be better to reword this as "Who can run fast?"
Anonymous hirashinBut do you ever say (b) "Who does run fast?"It would be better to reword this as "Who can run fast?"Are you a native English speaker?
hirashinI think native English speakers say either"Who runs fast?" or"Who can run fast".These are both correct English. There may be contextual reasons for choosing one over the other. Overall, the version with "can" would probably be more common.