Those are all possible. However, with the omission of "do they have", (c) feels more abbreviated, perhaps too much so for a question in an English test.
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GPYHowever, with the omission of "do they have"Hi GPY,
GPY(c) feels more abbreviated, perhaps too much so for a question in an English test.I won't write that one. They are beginners.
Tenacious LearnerThanks! Then, I should put it between parentheses.You mean write this?:
GPYHow long (do they have) until they have to be at the office?I don't recommend that.Hi GPY,
GPYNo, that isn't what I meant.Oops! Sorry I misunderstood you.