Final answer:
The phrase '(his grade suffering)' in the provided sentence is an example of a present participial phrase, describing 'his grade' and providing additional information about why Benjamin started going to the tutoring center.
Step-by-step explanation:
The portion in parentheses in the sentence "Benjamin, (his grade suffering,) started going to the tutoring center." is best identified as an e) present participial phrase. Present participial phrases start with a present participle, which is a verb ending in -ing that functions as an adjective. In this case, "suffering" is the present participle describing "his grade." The phrase provides extra information about Benjamin's grade and is not essential to the sentence's meaning; hence it is not an independent or dependent clause. It also does not function as a prepositional or infinitive phrase.