Wordwizard

a. Which one saw the accident, the tall boy or the short one?
b. Which saw the accident, the tall boy or the short one?

c. Which one saw the accident, the one called Peter or the one called John?
d. Which saw the accident, the one called Peter or the one called John?

e. Which one saw the accident, Peter or John?
f. Which saw the accident, Peter or John?

Which of the above sentences are grammatically correct?
Which are natural?

Many thanks.

In isolation, none sound natural. In isolation, they would all sound more natural reformulated with "who".

In the context of a dialogue, parts of the sentences can be made to work.

[At the scene of an accident, a police officer is talking to bystanders]
Officer: "Did any of you see the accident?"
Woman: "No, but I think one of those two lads over there saw it happen."
Officer: "Which one saw the accident, the tall one or the short one?"

Note that even with the context I have reformulated the sentence to make it sound natural.