Answer: a

Explanation:
Although till is often considered to be an abbreviation of until — and is frequently thought to be incorrect for its extra letter l and lack of apostrophe — it actually predates the longer word. People who want to use a less formal variant sometimes use ’til, but that’s not actually a word. Because we’ve got two perfectly good, equally correct words that mean the same thing, we don’t need to incorporate another into the language. (Merriam-Webster, I’m talking to you!)

See more about ’til, till and until on World Wide Words and motivated grammar.

Thanks to @divinewrite for the idea for this post. (He, in turn, thanks @code_and_prose for setting him up with a question.)

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