What is the meaning of the phrasal verb 'check in'?

[uta]

English Language Tests, Intermediate level

ESL/EFL Test #235 "Phrasal Verbs and Traveling", question 1

After you buy an airplane ticket, you must ......... once you arrive at the airport in order to confirm the flight and get a seat on the aircraft.

(a) check over
(b) check in
(c) check out
(d) check off

English Language Tests, Intermediate level

ESL/EFL Test #235 "Phrasal Verbs and Traveling", answer 1

After you buy an airplane ticket, you must check in once you arrive at the airport in order to confirm the flight and get a seat on the aircraft.

Correct answer: (b) check in
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What is the correct answer?

[Mister Micawber]

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It appears right above your question in your post: check in.
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Native English teacher at Mister Micawber's