<p dir="ltr" style="text-align: justify;"><span><span style="">The question is a simple sentence that follows the pattern: S + V. The subject is “A huge number of refund requests,” and the verb is the blank. The phrase “because of product defects and safety issues” is to add extra information to the sentence. </span></span></p><p dir="ltr" style="text-align: justify;"><span><span style="">Choice (A) </span><span style="">is occurred</span><span style=""> and Choice (D) </span><span style="">occurs</span><span style=""> are incorrect because both of them are singular verbs, but the subject of the sentence is plural. Choice (B) </span><span style="">are occurred</span><span style="">, though it’s a plural verb, is incorrect too because the verb “occur” is an intransitive verb and can’t be used in the passive voice. Thus, Choice (C) </span><span style="">occur</span><span style=""> is the correct answer. </span></span><br></p>