Sales invoices should be (a) numbered in sequence (b) prepared in triplicate (c) used as source documents for sales on account (d) All of these.

The answer is (d) All of these. Sales invoices should be numbered in sequence to keep track of them and ensure that none are missing. They should be prepared in triplicate so that the original can be sent to the customer, a copy can be kept for the seller's records, and another copy can be used for accounting purposes. Sales invoices are used as source documents for sales on account, meaning that they provide evidence of the transaction and serve as a basis for recording it in the accounting system. Therefore, all of these options are important for proper sales invoice management.