Final answer:
The correct answer is 1) part of accounts payable.
Step-by-step explanation:
A supplier invoice is a document issued by a supplier to a buyer, requesting payment for goods or services provided. It serves as a record of the transaction and includes details such as the quantity, price, and terms of the purchase.
Regarding the options provided in the question, a supplier invoice is indeed part of accounts payable, as it represents an outstanding liability owed by the buyer to the supplier. It is not an independent accounting function or a treasury function. While a supplier invoice is recorded in the general ledger department, it is not part of the department itself.