A thin ring of radius \(R\) lies in the \(xy\)-plane and is centered at the origin. The ring carries a non-uniform linear charge density given by \(\lambda(\theta) = \lambda_0 \cos(2\theta)\), where \(\theta\) is the azimuthal angle measured counterclockwise from the positive \(x\)-axis and \(\lambda_0\) is a positive constant. Which of the following expressions correctly gives the electric potential \(V(z)\) as a function of position \(z\) along the central \(z\)-axis, relative to \(V = 0\) at infinity?