Concept:Parentheses add extra information, and common phrases like "all day" should not be hyphenated or merged.
Rule / Clue:The phrase "all day" is a standard expression written as two separate words without a hyphen.
Explanation:Option A uses parentheses correctly around "all day" to clarify the duration.
Option B incorrectly adds a hyphen between "all" and "day".
Option C merges them into one word, which is not valid in English.
Therefore, only option A is correctly punctuated.
Answer:A. You studied (all day) for the grammar test.