Final answer:
The purchase invoice is the source document that should be entered into the accounting system, as it provides evidence of a purchase and includes important transaction details.
Step-by-step explanation:
The source document that would be entered into the accounting system from among the options provided is the purchase invoice. A source document is an original record containing the details to substantiate a transaction entered in an accounting system. For instance, a purchase invoice acts as evidence of a purchase made, and it includes detailed information about the transaction such as the description and quantity of goods or services, prices, discounts, date of the transaction, and the terms of payment.
Other documents like the sales order and purchase order are important in the business cycle, but they act as a confirmation of an order rather than evidence of a financial transaction that has occurred. The goods received note typically confirms receipt of goods but does not necessarily signify that a financial transaction has taken place. The purchase invoice, by contrast, is used to record actual financial transactions relating to purchases in the accounts payable ledger and later used to match payments made, making it the correct answer to the question.