Final answer:
In physics, when a circuit is only receiving 40 volts, the recommended action is to add more voltage sources or check for circuit faults. Increasing or decreasing resistance would not directly affect the voltage level received by the circuit.
Correct option is D. Check for circuit faults
Step-by-step explanation:
The subject of this question is Physics, specifically relating to electric circuits. When a circuit is only receiving 40 volts and a higher voltage is required, the most appropriate action to address the voltage issue would be to add more voltage sources (Option C). Increase resistance would lower the current flow given a fixed voltage, and decrease resistance might increase current flow but would not increase the actual voltage in the system. Therefore, adding more voltage sources is recommended to achieve the desired voltage level.
It is also important to check for circuit faults (Option D), such as a faulty connection or component, which might be causing a voltage drop in the circuit.