Answer:
While.
Step-by-step explanation:
A conjunction is a word that joins words, groups of words, clauses, or sentences. It acts as the linking word in the sentence.
A subordinating conjunction is a word that joins subordinating clauses in a sentence.
In the given sentence, "Addison Montgomery caught up on old issues of The New Yorker" is an independent clause while "she waited at the airport" is a subordinate or dependent clause. Therefore, to join the dependent clause to the main clause, the word "while" is used which is a subordinate clause.
Thus, the subordinate conjunction in the sentence is "while".