In the diagram, the arrow shows the movement of electric charges through a wire connected to a battery.

what causes the electric charges to flow from one end of the battery to the other?

a] a balance in electric potential
b] a balance in resistance
c] a difference in electric potential
d] a difference in resistance

So we want to know what causes the electric charges to flow from one end of the battery to the other like the picture shows. In the battery we have a positive and a negative charge that are separated. They give an electric potential difference also known as voltage. The voltage causes the charge (electrons) to move from the negative pole to the positive pole. That is called the electron flow and that is what is what is actually happening in the electric circuit. The image shows that the current flows from the positive pole to the negative. That is called "conventional current" and it is a matter of convention. Both are caused by the difference in electric potential. So the correct answer is C. 

2. they have negligible mass.