A long, solid cylindrical conductor of radius \(R\) carries a uniform current density \(J\) parallel to its central axis. A cylindrical cavity of radius \(a\) is drilled parallel to the conductor's axis, with its center displaced by a distance \(d\) from the central axis of the conductor, where \(d + a < R\). Which of the following expressions represents the magnitude of the magnetic field at any point inside the cavity?