According to Mario Pei, slang is common to English as well as to foreign languages (80). Many words that were once considered slang are now words of full, legitimate standing in our language. Some of these words are strenuous, spurious, clumsy, and bogus. Even Shakespeare introduced into current usage such slang terms of his day as hubbub, fireworks, fretful, (to) bump, and (to) dwindle. Pei estimates that about 90 per cent of all slang words eventually become permanent (Johns 12).
What type of language has the author used?
A. He is using informal language.
B. He is using formal language.
C. He is using many negative connotations.
D. He is using inconsistent language.

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.