$\begin {array} {1 1} (a)\;\large\frac{\mu_0Ni}{a} & \quad (b)\;\large\frac{\mu_0Ni}{b} \\ (c)\;\large\frac{\mu_0Ni}{2(b-a)}In \bigg( \large\frac{b}{a} \bigg) & \quad (d)\;\large\frac{\mu_0Ni}{(b-a)}In \bigg( \large\frac{b}{a} \bigg) \end {array}$