A thin flat disk of radius \(R\) lies in the \(xy\)-plane centered at the origin. The disk has a non-uniform surface charge density given in polar coordinates by \(\sigma(r, \theta) = \sigma_0 \left(\dfrac{r}{R}\right) \cos\theta\), where \(\sigma_0\) is a positive constant, \(r\) is the radial distance from the origin, and \(\theta\) is measured counterclockwise from the positive \(x\)-axis. Which of the following expressions gives the magnitude of the electric field at the origin?