<p dir="ltr" style="text-align: justify;">In the question, the blank connects two clauses, which means the blank is a conjunction, either a coordinating conjunction or a subordinating one. Therefore, (A) "during" should be eliminated since it is a preposition, not a conjunction. </p><p><span style="text-align: justify;">All the other three choices are subordinating conjunctions, but based on the meaning of the context, the correct answer is </span><strong style="text-align: justify;">(C) until after</strong><span style="text-align: justify;">.</span><br></p>
<p>The phrase "until after" is used to indicate that passengers should remain seated for the entire duration of the time period after the seatbelt light has been switched off. This means that passengers should not leave their seats until a certain point in time has passed.</p>
<p>The other options can be eliminated:</p>
<p>(B) "whether" is incorrect because it does not make sense in this context. The sentence is not asking passengers to make a choice or decision, but rather giving them an instruction to follow.</p>
<p>(D) "although" is incorrect because it suggests that passengers should remain seated even if the seatbelt light has not been switched off. This contradicts the intended meaning of the sentence, which is that passengers should only remain seated until the seatbelt light has been switched off.</p>