The correct answer is: Evidence as to the existence of the company's work in progress inventories derived from physical inspection by the auditor.

Evidence as to the existence of the company's work in progress inventories derived from physical inspection by the auditor is evidence originated by the auditor and, as such, is prima facie more reliable than the other options which are all examples of evidence obtained from other sources. In particular oral evidence as to the validity of an account receivable derived from a telephone confirmation of the balance by the customer would be more reliable if it was confirmed in writing and being from a third party is more reliable than client generated.