Final answer:
The adjectives in the sentence are 'big' and 'quiet,' which describe 'city' and 'suburbs' respectively. These words provide more detail about the nouns and modify them.
Step-by-step explanation:
In the sentence “three friends of mine live happily in the big city but I prefer the quiet suburbs,” the word 'big' describes the noun 'city' and the word 'quiet' describes the noun 'suburbs.' Both of these words are adjectives because they provide more information about the respective nouns. Adjectives modify nouns by giving some detail about an object's size, shape, age, color, origin, or material. It's interesting to note advice from Mark Twain, who suggested using adjectives sparingly to make writing more effective. Indeed, eliminating unnecessary adjectives often strengthens a sentence.