Which of the following actions is within state power as indicated by the 10th Amendment?

The action of setting traffic laws falls within state power as indicated by the 10th Amendment because the amendment reserves to the states all powers not specifically granted to the federal government or prohibited to the states by the Constitution. Traffic laws are concerned with the safety and regulation of movement within a state's borders, which is a typical exercise of state authority. States have the responsibility and authority to regulate activities, establish laws, and govern issues that directly affect their residents, such as transportation and vehicle operations.

In contrast, the establishment of postal services is primarily a responsibility of the federal government, as outlined in the Constitution. Conducting foreign affairs and regulating international trade are also powers reserved for the federal government, as these issues extend beyond state borders and require a uniform national approach. Thus, the ability to set traffic laws is a clear exercise of state power, aligning directly with the intent of the 10th Amendment to affirm state sovereignty in areas not specifically addressed by federal authority.