Choice "A" is correct. Attributes sampling is the statistical sampling method used when testing controls.
Choice "d" is incorrect. Variables sampling is used in substantive testing, not in tests of controls.
Choice "c" is incorrect. Stratified sampling is used in conjunction with variables sampling, and is therefore used in substantive testing, not in tests of controls.
Choice "b" is incorrect. Ratio estimation is used in conjunction with variables sampling, and is therefore used in substantive testing, not in tests of controls.
