Choice "A" is correct. Attribute sampling is used to test controls. Inspecting employee time cards for proper approval by supervisors is a test of controls. Controls often relate to authorization, validity, completeness, accuracy, appropriate classification, accounting in conformity with GAAP, and proper period. Look for these terms in identifying which option is a test of controls. Words such as account balance, amount, valuation, presentation, and disclosure are more likely to relate to substantive tests.
Choice "b" is incorrect. Selecting accounts receivable for confirmation of accounts balances is a substantive test.
Choice "c" is incorrect. Making an independent estimate of the amount of a LIFO inventory is a substantive test.
Choice "d" is incorrect. Examining invoices in support of the valuation of fixed asset additions is a substantive test.