Time-Frame Test: Ask of each statement: does it describe ONE moment or a PERIOD of activity?
The Balance Sheet is the only one that answers 'as of [date]' — it lists Assets, Liabilities, and Equity at a single instant (Assets = Liabilities + Equity).
Income Statement, Cash Flow Statement, and Retained Earnings Statement all summarize what happened OVER a period (a quarter or year), so none of them is a single-date snapshot.