Explanation

(b)
Value of magnetic moment depends upon number of unpaired electrons. All except ${Ti}^{3 +}|3d^{1}$| have either fully filled $d$-subshell ($i.e$., $Zn^{2 +},\ Cu^{+})\$or empty $d$- subshell ($ie$, $Sc^{3 +}$). As such only ${Ti}^{3 +}$ has a net value of magnetic moment.
Magnetic moment of ${Ti}^{3 +}$=$\sqrt{n(n + 2)}\ BM$
=$\sqrt{1(1 + 2)}\ BM$
=$\sqrt{3}$ =1.73$\ BM$