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 total distance that martina travels that day is

(distance from home to mom's office) plus (distance from mom's office to school)

plus (distance from school to library) plus (distance from library to friend's house)

plus (distance from friend's house to martina's house).

(we don't know what any of these distance actually is.)

a is the best answer your welcome