Consider this example :
"I saw one of the Jones brothers yesterday"
"Yes?Which one/Whom did you see?
I think "whom" is correct because I am referring to people, but which one also feels correct to me. Which of these is better?
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which one is the usual way to ask that question referring to one of several possibilities. whom, or nowadays who (the objective case form whom is falling out of use in speech) is a more general question: tell me who you saw not tell me which one of them you saw. Only from context would it be understood to mean "which of them".