Which of the following audit tests would be regarded as a test of controls?

Which of the following audit tests would be regarded as a test of controls?

    a.    Comparison of the inventory pricing to vendors’ invoices.
    b.    Tests of the signatures on canceled checks to board of directors’ authorizations.
    c.    Tests of the additions to property, plant, and equipment by physical inspections.
    d.    Review of the specific items making up the balance in a given general ledger account.