Final answer:
The 'closeness' of two parallel conducting plates refers to the ratio of plate separation to plate area, reflecting the importance of relative proportions to minimize edge effects in an electric field.
Step-by-step explanation:
When discussing the electric field between two parallel conducting plates, the concept of 'close together' typically implies that the edge effects are minimized. This notion of closeness is not solely about the actual plate separation but more about the ratio of plate separation to plate area. A smaller separation between the plates means less space for the fringing effects to occur, and larger plates can further reduce these edge effects due to the relative proportion where the central area (showing uniform electric field) occupies most of the plate area. Therefore, in the scenario presented, the most accurate answer is b) Ratio of plate separation to plate area.
It is important to remember that as the ratio becomes smaller, the field becomes more uniform, except very close to the edges. In practical considerations for the design of capacitors, we aim for a configuration where the ratio of plate separation to plate area is small to ensure a more uniform electric field, thus minimizing the impact of non-uniform fields near the edges or the 'edge effects'.