Choice "A" is correct. By ensuring that credit approval is obtained before goods are shipped to customers, the auditor is testing management's assertion that accounts receivable are collectible (valuation or allocation).
Choice "d" is incorrect. Ensuring that credit approval is obtained before goods are shipped does not support the completeness assertion.
Choice "b" is incorrect. Ensuring that credit approval is obtained before goods are shipped does not support the existence assertion.
Choice "c" is incorrect. Ensuring that credit approval is obtained before goods are shipped does not support the rights and obligations assertion.