The correct answer is D) chemical agents. The use of chemical agents to clean up oil spills is generally not allowed under U.S. Coast Guard regulations. This is because chemical dispersants and other chemical agents can be harmful to the environment and marine life, and their use is typically restricted or prohibited except in certain emergency situations. In contrast, the use of suction equipment, booms, and skimmers are generally approved and preferred methods for containing and cleaning up oil spills, as they physically remove the spilled oil without introducing additional chemicals into the environment.