"Correct answer" had "1" as a numeral which is definitely wrong. Having "one" at all is a little awkward but still acceptable.
quite agree, the inclusion of 'one' in this sentence is optional in english.
when using 'which' as a question word (interrogative pronoun) the idea that you are identifying 'one' thing from a group of things is implied, and the 'one' need not be explicitly stated.
i can see that 'which one' does have the benefit for learners of english to clarify the use of 'which' here as an interrogative pronoun. however it would be good to add 'which is your choice?' as an acceptable answer here.