Choice "D" is correct. Perfection has little effect on a buyer in the ordinary course of business (such a buyer takes subject to a perfected security interest only if the buyer knows that the sale violates the security agreement).
Choice "a" is incorrect. Perfection gives the secured party superior rights in the collateral as against most later creditors.
Choice "c" is incorrect. If the bankruptcy is filed after perfection, the secured party will have priority in the collateral as against the trustee in bankruptcy because the trustee is treated as a lien creditor as of the day the bankruptcy petition is filed, and a prior perfected security interest has priority over a subsequent lien creditor.
Choice "b" is incorrect. Subsequent judgment creditors have lower priority in collateral than a secured creditor who has perfected a security interest in the collateral.