C) Its vocabulary and grammar.
The vocabulary or choice of words, as well as the way they're put together, is what mainly differentiates informal writing from a formal one. In this excerpt, the chosen vocabulary, the adequately use of grammar, the absent of colloquialism, contractions and first-person pronouns like "I" and "We" (It isn't narrated in first-person) are clear indicators that the passage is an example of formal writing.
