b
(b) Let $A = \left[ {\begin{array}{*{20}{c}}1&2&3\\0&1&2\\0&0&1\end{array}} \right]$ $ \Rightarrow $ $|A| = 1\,(1 + 0) = 1$
$Adj\,(A) = \left[ {\begin{array}{*{20}{c}}{{A_{11}}}&{{A_{21}}}&{{A_{31}}}\\{{A_{12}}}&{{A_{22}}}&{{A_{32}}}\\{{A_{13}}}&{{A_{23}}}&{{A_{33}}}\end{array}} \right]$
$Adj\,(A) = \left[ {\begin{array}{*{20}{c}}1&{ - 2}&1\\0&1&{ - 2}\\0&0&1\end{array}} \right]$
$ \Rightarrow $ ${A^{ - 1}} = \frac{{Adj\,(A)}}{{|A|}} = \left[ {\begin{array}{*{20}{c}}1&{ - 2}&1\\0&1&{ - 2}\\0&0&1\end{array}} \right]$.