Leaving to one side whether “who” is the subject or the object of the sentence, to the best of my knowledge all four questions below are considered grammatical and acceptable ways of asking who the person with Sarah was:
a) “Who was Sarah with?”
b) “Who was with Sarah?”
c) “Whom was Sarah with?”
d) “With whom was Sarah?”
I would like to know which form is considered the most natural in speech, I think it's a toss up between a) and b), and which the most “correct” from a native speaker's point of view. Can it please be someone who is currently living in an anglophone country.
I would also like to know if the following are acceptable. I'm not sure how to classify this type of question, the speaker is asking for confirmation because they either didn't hear the person's name or they cannot believe the statement is true, e.g.
Mike: Yesterday I saw Sarah with Adam (Lisa's ex-husband).
Mary: e) Sarah was with whom?
f) Sarah was with who?