Book-keeping entries record transactions in journals and ledgers, not financial statements.

Steps:

• Define book-keeping entry as a record of transactions in accounting books like journals, ledgers, or cash books.
• Evaluate option A: It's a direct entry in the cash book, a book-keeping record.
• Evaluate option B: It's a ledger entry in the supplier's account, part of book-keeping.
• Evaluate option C: Entering expenses in the income statement is part of financial reporting, not initial book-keeping.
• Evaluate option D: It's a journal entry in the sales returns book, a book-keeping record.

Why C is correct:

• Book-keeping involves recording raw transactions in journals and ledgers; the income statement is a summarized financial statement prepared after book-keeping, per double-entry accounting principles.

Why the others are wrong:

• A: Cash book entries are fundamental book-keeping for tracking receipts.
• B: Supplier account entries are ledger postings, core to book-keeping.
• D: Sales returns journal records returns as part of transactional book-keeping.

Final answer: C