d
(d) ${A^2} = A.\,A = \left[ {\begin{array}{*{20}{c}}1&a\\0&1\end{array}} \right]\,\left[ {\begin{array}{*{20}{c}}1&a\\0&1\end{array}} \right] = \left[ {\begin{array}{*{20}{c}}1&{2a}\\0&1\end{array}} \right]$
${A^3} = A.\,{A^2} = \left[ {\begin{array}{*{20}{c}}1&a\\0&1\end{array}} \right]\,\left[ {\begin{array}{*{20}{c}}1&{2a}\\0&1\end{array}} \right] = \left[ {\begin{array}{*{20}{c}}1&{3a}\\0&1\end{array}} \right]$
${A^4} = A.\,{A^3} = \left[ {\begin{array}{*{20}{c}}1&a\\0&1\end{array}} \right]\,\left[ {\begin{array}{*{20}{c}}1&{3a}\\0&1\end{array}} \right] = \left[ {\begin{array}{*{20}{c}}1&{4a}\\0&1\end{array}} \right]$.