Answer
The magnitude of the electric field at $r = \frac{R}{2.00}$ is zero.
Work Step by Step
We can draw a Gaussian cylinder of radius $r = \frac{R}{2.00}$ that has the same axis as the charged cylinder.
Since the enclosed charge in the Gaussian cylinder is zero, the magnitude of the electric field at $r = \frac{R}{2.00}$ is zero.
