Answer
Choice D.
Work Step by Step
We see that $E_{max}$ is proportional to $\frac{1}{f}$, so doubling the frequency changes the maximum field by a factor of $\frac{1}{2}$.
The intensity is proportional to $E^2_{max}$, so changing $E_{max}$ by a factor of $\frac{1}{2}$ will change the intensity by a factor of $(\frac{1}{2})^2=\frac{1}{4}$.
