a
det $\left[ {{{\left( {I + B} \right)}^{50}} - 50B} \right]$
$=$ det $[ {\,^{50}}{C_0}I + {\,^{50}}{C_1}B + {\,^{50}}{C_2}{B^2} + {\,^{50}}{C_3}{B^3} + ... +$ $ {\,^{50}}{C_{50}}{B^{50}}{B^{50}} - 50B]$
{All terms having ${B^n},2 \le n \le 50$
will be zero because given that ${B^2} = 0$ }
$=$ det $\left[ {I + 50B - 50B} \right]$$=$ det $[I]=1$