<p>Airline passengers must <u>check in</u> three hours before departure time.</p><div data-answer="a"><span>The correct answer is (A) check in. </span></div><div data-answer="a"><br></div><div data-answer="a"><span>When traveling by air, passengers are required to check in before their flight. This involves presenting their ticket or reservation information, providing identification, and receiving a boarding pass. Checking in allows the airline to verify the passenger's information, assign them a seat, and ensure that they are present for the flight. </span></div><div data-answer="a"><br></div><div data-answer="a"><span>(B) </span><span>"Check out" is not the correct answer because it means to leave a place, such as a hotel or a store, after completing a transaction. </span></div><div data-answer="a"><span>(C) </span><span>"Take in" means to absorb or understand something, and it is not relevant to the context of airline travel. </span></div><div data-answer="a"><span>(D) </span><span>"Keep up" means to maintain or stay informed about something, and it does not apply to the action of checking in before a flight.</span></div>