Choice "A" is correct. A buyer in the ordinary course of business is one who buys from a merchant's inventory. Such a buyer has the highest priority in the collateral and takes free of a perfected security interest regardless of the buyer's knowledge of the perfected interest unless the buyer knows that the sale is in violation of the security agreement.
Choice "b" is incorrect. Stockholders are owners of the business and have no particular priority in the assets of the business that have been pledged as collateral in a secured transaction.
Choices "d" and "c" are incorrect. A subsequent lien creditor or bankruptcy trustee has lower priority than a buyer in the ordinary course of business.