<p dir="ltr" style=""><span><span style="">This question is a complex sentence. The main clause “The toothpaste samples ________ back to the manufacturer” is followed by an adverb clause “when it was discovered that a number of the products’ cases were damaged.”. </span></span></p><p dir="ltr" style=""><span><span style="">Let’s focus on the main clause, “The toothpaste samples _______ back to the manufacturer.” The blank should be filled with a verb, so (D) "</span><span style="">being sent"</span><span style=""> is incorrect since it can’t be used as a verb in a sentence. </span></span></p><p dir="ltr" style=""><span><span style="">Of all the three other choices, (B) "</span><span style="">were sent"</span><span style=""> is the correct answer because, based on the meaning of the context, the samples should be sent to the manufacturer instead of sending something else back to them. </span></span></p><p dir="ltr" style=""><span><span style="">(A) "</span><span style="">were sending" </span><span style="">and (C) "</span><span style="">sent" </span><span style="">are incorrect because they are both verbs in the active voice, with the former in the past continuous tense and the latter in the simple past tense. </span></span></p>