Final answer:
The reason that would not be a valid reason to increase the extent of substantive testing is if auditors have time to test more items. (option c).
Step-by-step explanation:
In audit, the extent of substantive testing refers to the number of items that will be tested. Increasing the extent of substantive tests means testing more items from a population. Of the given options, the reason that would not be a valid reason to increase the extent of substantive testing is c) Auditors have time to test more items. The extent of substantive testing is determined by the risk of material misstatement, the presence of qualitative factors suggesting errors, and the strength of internal controls.