<p dir="ltr" style=""><span><span style="">This question is a compound sentence connecting two independent clauses. The first independent clause is “Last week, the manager sent a memo requesting all the workers must send their tax documents to the accounting department by the end of this week, and the second independent clause is “many of them missed the deadline.” In other words, the blank should be filled with a coordinating conjunction to connect these two independent clauses. </span></span></p><p>(A) "either" and (C) "by" are eliminated because they can’t act as coordinating conjunctions. <br></p><p>(B) "yet" is the coordinating conjunction. The phrase "yet" is used in this context to indicate that despite the manager's request, many of the workers still missed the deadline. It suggests a contrast between what was expected (that all workers would send their tax documents) and what actually happened (many of them missed the deadline).</p><p dir="ltr" style=""><span><span style="">(D) "</span><span style="">so"</span><span style=""> is a coordinating conjunction, but it is incorrect because </span></span>it is used to show a consequence or result, which is not the intended meaning in this sentence. </p>