The correct answer is C) National Cargo Bureau. The National Cargo Bureau is the organization responsible for certifying that all preparations have been made and all regulations observed prior to loading a cargo of bulk grain. This is because the National Cargo Bureau is the designated authority that oversees the safe loading and transportation of bulk cargoes, including grain, to ensure compliance with applicable regulations and industry standards. The other options are incorrect because the master of the vessel (A) is responsible for the overall operation of the ship, but not specifically for certifying the cargo loading process. The U.S. Coast Guard (B) and the American Bureau of Shipping (D) have other regulatory and classification roles, but do not have the specific authority to certify the readiness of a bulk grain cargo.