By definition under RCRA hazardous wastes are:

Hazardous wastes under the Resource Conservation and Recovery Act (RCRA) are defined as solid wastes that pose a potential threat to human health or the environment. This includes a wide range of materials that can be solid, liquid, or contained gases, but the regulatory framework categorizes them primarily as solid wastes due to the way they are managed and regulated.

By identifying hazardous wastes as solid, RCRA establishes a baseline for the management, safe disposal, and treatment of these materials. This definition also encompasses substances that may be found in various physical forms, thus aligning with the U.S. Environmental Protection Agency's regulatory controls.

Other options do not accurately capture the regulatory definition of hazardous waste. Natural is not specified in the RCRA context regarding hazardous waste, nor is gray, which lacks a clear connection to hazardous waste classification. The choice of "None of the above" would imply that hazardous wastes do not align with any of the given descriptors, which would be inaccurate in the context of RCRA definitions, reinforcing why the primary definition as solid is the correct understanding of hazardous waste classification.