According to U.S. regulations, which of the following does not qualify as a discharge of oil?

The reasoning for the correct answer lies in the context of U.S. regulations governing the discharge of oil into navigable waters. Discharging oil into the sea for pollution research is permitted under specific regulatory frameworks and is not considered an illegal or unauthorized discharge. These discharges are typically conducted with strict guidelines aimed at understanding and mitigating pollution impacts, thereby contributing to environmental protection efforts.

In contrast, the other scenarios listed involve illegal or unintentional discharges of oil that do not comply with regulatory standards. For instance, leakage from a transfer hose, spilling oil on the deck leading to its flow overboard, and pumping oil overboard through fixed piping systems are all examples of unauthorized discharges that can cause environmental harm. These actions violate regulations intended to minimize the introduction of pollutants into water bodies. Thus, option D stands out as it is an authorized action under certain conditions, distinguishing it from the other choices which represent typical violations of oily discharge regulations.