The primary rule of double-entry book-keeping is:

Double-entry bookkeeping is the foundational system for modern accounting. Its core principle is that every financial transaction has two equal and opposite effects, recorded as a debit in one account and a credit in another. This ensures the fundamental accounting equation (Assets = Liabilities + Equity) always remains in balance. While Option A is the accounting equation itself, Option B describes the mechanism by which that equation is maintained. Options C and D are specific accounting principles (Revenue Recognition and Matching Concept), and Option E is the Business Entity Concept.