Choice "A" is correct. If it would take less time or be more efficient to perform substantive tests than it would to perform tests of controls, and if there is no other reason to test controls (i.e., if there is not a high degree of electronic processing), the auditor would not be likely to test controls.
Choice "d" is incorrect. In order to assess control risk at a low level, tests of controls would need to be performed. However, if it is more efficient to perform substantive tests, the auditor would not be likely to test controls.
Choice "b" is incorrect. If it would take less time to perform substantive tests than it would to perform tests of controls, and if there is no other reason to test controls (i.e., if there is not a high degree of electronic processing), the auditor would not be likely to test controls.
Choice "c" is incorrect. Substantive procedures are always be necessary for all assertions relevant to material transaction classes, account balances, and disclosures. Assuming inventory is material, tests of controls alone would not be sufficient.