I would go with nature because thats what it sounds like to me hope this helps<span />
The prepositional phrase in this sentence is c. "to your sentence structure", because it is postmodifying an infinitive phrase (to bring complexity), which has the same value, in this case, as a noun phrase. Besides it is composed by a preposition (to) and a noun phrase (your sentence structure).
Option a. is a verb phrase as well as option b. In the same way, letter d. is not a prepositional phrase as it is an adjective phrase used in the comparative form.
I would say the answer is "harbor". This word is usually used as a noun; but in this case, it is a verb that means to provide shelter to someone or something, to keep someone safe, but also to have something in mind and think about it. This word is purposefully chosen and fits into Whitman's well-intended, philanthropic and inclusive worldview.
It tells, how different cultures are, or how similar they can be. lol
