What should be done to copper before applying it in a body of water?

Prior to applying copper in a body of water, it is essential to wait 24 hours to allow any metallic smell to dissipate. This step is important because it ensures that the copper's presence will not cause immediate toxic reactions in aquatic organisms. The metallic smell can be an indication of high concentrations of copper, which may be harmful to fish and other aquatic life. By allowing time for the smell to dissipate, you are taking measures to minimize potential toxicity risks and ensure that the application of copper aligns with best practices for maintaining water quality and protecting the ecosystem.

The other choices are less relevant or effective in preparing copper for application. For instance, increasing the temperature or stirring vigorously does not specifically address the safety and environmental concerns related to the odor and potential toxicity of copper in aquatic environments. Mixing with other herbicides can also pose risks, as it could lead to unforeseen chemical reactions that might increase toxicity levels or decrease effectiveness. Therefore, waiting for the metallic smell to dissipate is the most appropriate action to ensure safety when applying copper in aquatic settings.