(a)
Since, $\left( 1 + \frac{1}{n} \right)^{n} < 3\ for\ \forall n \in N$
Now, $\frac{{(1001)}^{999}}{{(1000)}^{1000}} = \frac{1}{1001} \bullet \left( \frac{1001}{1000} \right)^{1000}$
$= \frac{1}{1001}\left( 1 + \frac{1}{1000} \right)^{1000} < \frac{1}{1001} \bullet 3 < 1$
${(1001)}^{999} < {(1000)}^{1000}$
$\therefore\ B < A$