a
(a) Here $AB = \left[ {\begin{array}{*{20}{c}}i&0\\0&i\end{array}} \right]\,\left[ {\begin{array}{*{20}{c}}0&{ - i}\\{ - i}&0\end{array}} \right] = \left[ {\begin{array}{*{20}{c}}0&1\\1&0\end{array}} \right]$
and $BA = \left[ {\begin{array}{*{20}{c}}0&{ - i}\\{ - i}&0\end{array}} \right]\left[ {\begin{array}{*{20}{c}}i&0\\0&i\end{array}} \right]\, = \left[ {\begin{array}{*{20}{c}}0&1\\1&0\end{array}} \right]$
Since $AB = BA,$ therefore $(A + B)(A - B) = {A^2} - {B^2}$.