A long, thick-walled hollow cylindrical solenoid has inner radius \(a\) and outer radius \(b\). The solenoid carries a non-uniform azimuthal volume current density \(\vec{J} = J_0 \left(\dfrac{r}{a}\right) \hat{\theta}\) within its wall (\(a \le r \le b\)), where \(J_0\) is a positive constant and \(r\) is the radial distance from the central axis. Outside the solenoid (\(r > b\)), the magnetic field is zero. Which of the following expressions gives the magnitude of the magnetic field \(B(r)\) inside the solenoid wall, in the region \(a \le r \le b\)?