(d)
$f(0) = \lim_{x \rightarrow 0}\frac{\left( e^{x} - 1 \right)^{2}}{\sin{\left( \frac{x}{a} \right)\log\left( 1 + \frac{x}{4} \right)}}$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \lim_{x \rightarrow 0}\ \left( \frac{e^{x} - 1}{x} \right)^{2}.\ \frac{\frac{x}{a}.a}{\sin\frac{x}{a}}.\frac{\frac{x}{4}.4}{\log\left( 1 + \frac{x}{4} \right)} = 12$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ 1^{2}.a.4 = 12$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ a = 3$