The main thing that this question is asking is which sentence does not have a formal tone. A formal tone for research writing usually means using more professional (instead of casual, everyday) language and free of your own opinions.
This means choice D is inappropriate because it is opinionated. Using "I think" is not formal tone.
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Answer: D) I think one of the most frustrating things about owning a cat is that they don't obey many commands.
