Research suggests that, when facing a threat, the presence of one's partner

The answer is: makes people feel calmer and less nervous, since they feel more relaxed in the presence of their partner.
People tend to see their partner as someone that will always on their side and will back them up whenever they're facing a threat.
This means that by the existence of our partner alone, we will obtain some sense of security in every situation

truly perplexes them.

Explanation:

Expository essays requires thorough research and evaluation of a particular topic which provides a good argument regarding the topic at the end.

It requires full attention in unfolding the problems of a topic as the writers must provide a concise argument at the end.

Therefore, the person who tries to write an exploratory essay must always choose a topic that truly perplexes them. They should challenge themselves in learning and unfolding the truth about the subject.