The simulation shows the equipotentials. Where is the magnitude of the electric field greatest?

1. At the top
2. At the bottom
3. In the middle
4. On the left
5. On the right
6. The field has the same magnitude everywhere
7. There is not enough information to decide

The field is strongest at the top, where the equipotentials are closer together. You can see this by adding a positive test charge to the simulation and testing to see where it feels the strongest force.