Voltage sources are used to sustain
a. the flow of protons
b. chemical reactions
c. resistance
d. the movement of electric charge

d. the movement of electric charge

Explanation:

Voltage source is used to provide constant voltage difference across the ends of a given conductor where it is connected

Now when voltage source is connected across the conductor we can say that due to constant voltage difference there would be a constant current will maintain in the conductor

It is given by

now since the voltage difference and resistance both are constant so current will be same in the circuit

So here the flow of charge will remain constant due to voltage source

So correct answer would be

d. the movement of electric charge

the motion of a pendulum is a classic example of mechanical energy conservation. a pendulum consists of a mass (known as a bob ) attached by a string to a pivot point. as the pendulum moves it sweeps out a circular arc, moving back and forth in a periodic fashion.