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The primary role of a rheostat in a circuit is to adjust current flow. A rheostat is a type of variable resistor that allows for the control of the amount of current that can flow through a circuit. By changing the resistance within the circuit, a rheostat can increase or decrease the current according to the desired requirement. This feature is particularly useful in applications where the brightness of a light or the speed of a motor needs to be adjusted, providing flexibility and fine-tuning in electrical systems.

While other options suggest functions that are related to electrical handling, they do not accurately represent the specific purpose of a rheostat. For instance, converting AC to DC involves rectification and requires different components, while controlling voltage may involve other devices like voltage regulators. Storing electrical energy typically pertains to capacitors and batteries, not to the direct function of changing current as a rheostat does.