The correct answer is: To ensure that all credit notes received are recorded in the general and payables ledger

Rationale: All the other aims relate to the control system relating to receivables and sales.

Pitfalls: You have to get straight in your mind which transactions and controls relate to payables/purchases and which to receivables/sales.

Ways in: Careful reading of the question should allow you to eliminate 'To ensure that goods and services are only supplied to customers with good credit ratings' and 'To ensure that potentially doubtful debts are identified' immediately (sales/supply, not purchases).

You then have to think how credit notes work - but the fact that 'To ensure that all credit notes received are recorded in the general and payables ledger' mentions the 'purchase' ledger, and 'To ensure that all credit notes that have been issued are recorded in the general and receivables ledgers' the 'sales' ledger should lead you to the right answer.