What have many political scientists called laboratories of democracy? a. congressional committees b. local governments c. the executive agencies of the federal government d. the states e. special districts?

the states

The term “Laboratories of democracy”  is popularized by U.S. Supreme Court Justice Louis Brandeis to describe how a state may function as a laboratory where it can   do social and economic experiments without risk to the other states. If they work, then other states will use them. And perhaps even the entire country, will use them.

.elections are held to determine who serves in congress.