Answer:
The answer is:<u> A prepositional phrase. </u>
Explanation:
It is often confused these two terms, however, both are different, it is clear that both start with the particle "TO", however, as a infinitive, it should be the "To" plus a verb, whereas, a prepositional phrase which is a "to" plus a noun. In this example "research" could either be a verb or a noun, however, if you put it in both ways, you realize that "to" plus the verb, doesn't really make sense. But if you use it as as noun, and as a prepositional phrase, the equivalent will be "In order to research a topic, I use the reference section".
