I know you are speaking about the structure in indirect speech, Abdullah, and my recommendation is to think about the direct question and the answer to it to decide whether the wh-word is the subject or the subject complement of the question. If it is the subject, there will no inversion; if it is the subject complement, there will be inversion.
My point is that (A) (the most usual of the three, in my opinion), (B) and (C) are all grammatical and will be respectively used according to the type of direct question being asked. I hope you have understood that "which" can stand alone and does not need the pronoun "one" after it if it works (as is possible) as an interrogative pronoun rather than as an interrogative determiner. That said, let's see the possible direct questions for these reported versions:
A. Hana asked which was her seat.
This comes from: A'. 'Which is my seat?,' Hana asked. To which the usher possibly answered: A''. 'This is your seat, madam.' or A'''. 'The one over there is your seat, madam.'
As you can see, "which" is replaced with the subject "this" or the subject "the one over there" in the answers, and is therefore the subject of the question, so no inversion is required when the question is reported.
B. Hana asked which seat was hers.
This comes from: B'. 'Which seat is mine?,' Hana asked. To which the usher must have answered: B". 'This/That seat is yours, madam.'
In (B), "which seat" is the subject of the question (therefore, no inversion), as mirrored by "This/That seat" (also the subject) in the answer.
And now we get to the most unusual and complicated one:
C. Hana asked which her seat was.
For this reported question to work, we have to imagine a direct question (and an answer) where the word or phrase answering "which" is the subject complement (thus requiring inversion). As you know, in reported speech we sometimes simplify longer texts (for example, I'm very sorry for being so late can be reported as He apologized for his delay/late arrival). I think (C) has to be the reported version of a question in which "which of or among different seats" is implied:
C': 'Sir, I'm confused. Is my seat this one here or the one over there?'
In this question, "my seat" is the subject and "this one here or the one over there" is the subject complement. If we want to reflect this in reported speech, we can use (C), where "her seat" is the subject and "which" asks about the subject complement (therefore, we have inversion). A possible answer could be:
C''. 'Your seat is the one over there.'