Final answer:
The source documents for credit purchases are suppliers' invoices and credit notes. These serve as proof for financial transactions related to purchases made on credit.
Step-by-step explanation:
The source documents for credit purchases are A) suppliers' invoices and credit notes. These documents provide proof of the transactions that have taken place and are essential for maintaining accurate financial records. Suppliers' invoices serve as a request for payment for goods or services provided on credit, while credit notes are issued to correct any billing errors or to record returns or allowances on goods purchased on credit.
Invoices and credit notes to customers are related to sales, not purchases. Delivery notes are evidence of delivery, but do not necessarily relate to payment terms. Goods received notes confirm receipt of goods but again, do not pertain to the financial transaction terms.