Answer:
What makes this passage an example of formal rather than informal writing is:
its vocabulary and grammar.
Step-by-step explanation:
The passage we are analyzing here uses formal language. It is correct from a grammatical point of view, which means there are no structural mistakes such as problems with subject-verb agreement or with the relationship between a pronoun and its antecedent, for example. The vocabulary, that is, the words used in the passage, is also formal. We notice no use of slang at all. Instead, uncommon words such as "indelible", which we do not use on a daily basis, are employed in the passage.