At the point of maximum flux, the electric field is directed normal to the plane of the circular loop.
The electric flux may be calculated as,
Φ = E * A
Where we assume that the electric field, E, is uniform over the area of the circle, A.
We may solve for an unknown electric field magnitude as,
E = Φ / A
Where we are told that the flux is: Φ = 5.18 E5 N m^2 / C,
and we can calculate the area of the loop as,
A = pi * (D/2)^2 = (3.1415) * (0.48 m / 2)^2 = .1809504 m^2
Therefore,
E = (5.18 E5 N m^2 / C) / (.1809504 m^2)
E = ~2.86 E6 N/C
