I immediately understood that this question is about how work done to get a test charge from one equipotential line to another doesn’t depend on the path taken but this diagram seems a little strange to me. I only understand the part with a and b where there is a dashed circle with I and II by it. I’m assuming that path I represents the shorter looking path between a and b while path II represents the longer looking path (please correct me if I’m wrong).
What I don’t understand is why ra and rb are included; I’m assuming ra represents the distance between the source charge and any point (like point a?) on the first equipotential line aka the smaller circle and rb represents the distance between the source charge and any point (like point b?) on the second equipotential line aka the larger circle. They seem irrelevant but I could be mistaken.
Also what’s with path III? If I’m not mistaken there are 2 equipotential lines here: the smaller dashed circle and the larger dashed circle. Going from any point on the smaller circle to the larger circle or vice versa will result in the same magnitude of potential difference. So why specify two points?
Also I’m confused about the explanation about electric potential and electric potential energy. Why does electric potential depend on the source charge and EPE depend on the test charge?