Goal: Reasoning about electric fields
Source: 283 ring, E on axis
A ring
of radius R with charge +Q (uniformly distributed) is positioned as
shown. What is the electric field at a point on the axis, a distance x
from the origin?

None of the above.
Commentary:
Answer
(4) Discuss how the form of the field can be reasoned from symmetry and units. Together with limiting value as x goes to zero, this uniquely singles out one answer.
A good followup activity is to have students sketch a graph of the field and potential along the xaxis.