Choice "C" is correct. Tracing from invoices to shipping documents would provide evidence that sales billed to customers were actually shipped. An invoice for which the corresponding shipping documents could not be located might be indicative of fictitious sales (i.e., sales that were recorded but never actually shipped).
Choice "b" is incorrect. The auditor would need to start with shipping documents and trace to invoices to ensure that shipments were properly billed.
Choice "d" is incorrect. An invoice may exist for which no entry was made in the accounts receivable subsidiary ledger. Therefore, the auditor would need to trace from entries in the accounts receivable subsidiary ledger (and not from invoices) to shipping documents, to obtain evidence that recorded receivables were for sales actually shipped.
Choice "a" is incorrect. Tracing from invoices to shipping documents would not necessarily indicate when a duplicate shipment was made, as the auditor would not necessarily realize that two sets of shipping documents related to the same invoice.