GMAT Sentence Correction (SC) | Re: Several of the drownings that occurred last summer

Logic: we have a noun phrase as the subject and need a simple past tense verb for the same. ‘Which’ and ‘that’ in A and B turn the sentence into fragments while the present tense ‘are’ is inappropriate in C. D provides the correct direct verb ‘were caused’ while ‘being careless’ in E is wrong as it is neither used as part of a noun nor used in passive voice.
Therefore D

---