Parallel structure uses the same grammatical structure for items in a list. The items in this list are all objects of the preposition "by". Objects of the preposition are nouns or pronouns. These verbs must all be gerunds, because gerunds are verbs in noun form. All gerunds end in /ing/. For example, "breeding", "raiding", and "controlling". Using a different form of the verb or changing to a phrase doesn't keep the same grammatical structure needed for parallelism.
The answer is B. Controlling Trade. I have taken the test. Tip: Try to talk more formally it is more beneficial, and if you could speak more formal, you would be able to be understood better.
