Just by looking at this question, and the answer choices, you can determine that the least formal answer is A. Kicking back. This is because when you think of formal, you think of respectful and professional. Answers "C," and "B," are things that one may say in a formal/professional way to someone. However, people don't normally use the term, "kicking back," as a formal use of speech.
An adjectival phrase or adjective phrase consists of a group of words that describes a noun or a pronoun. In the given sentence, the adjective phrase is, option C. Those old. The noun it describes is, "Patio Chairs".
