A thin, nonconducting spherical shell of radius \(R\) is centered at the origin. The shell carries a non-uniform surface charge density given by \(\sigma(\theta) = \sigma_0 \cos\theta\), where \(\sigma_0\) is a positive constant and \(\theta\) is the polar angle measured from the positive \(z\)-axis. Which of the following expressions gives the electric potential \(V(z)\) at a point on the positive \(z\)-axis at a distance \(z > R\) from the origin, relative to \(V = 0\) at infinity?