Answer:
B. He is using formal language.
Step-by-step explanation:
In the given passage, the speaker is talking about how "many words that were once considered slang are now words of full, legitimate standing in our language". This has led to many of the common everyday words becoming legitimate words of their own, even used in many formal writings and even used by Shakespeare. The narrator also states that he (Shakespeare) even "introduced" some slang words such as "hubbub, fireworks, fretful, (to) bump, and (to) dwindle". And considering the text, the form of writing cites sources, making it a formal one. The author doesn't use words that are used loosely, thus eliminating informal language.
Again, there are no negative words that suggest any negative connotation, and the flow of the passage is consistent, thus leaving us with just the second option.
Thus, the author used formal language in the text.