(b)
Since, $\alpha$ is a repeated root.
$\therefore\ \ \ \ \ ax^{2} + bx + c = a(x - \alpha)^{2}$
Now, $\lim_{x \rightarrow \alpha}\frac{\sin{(ax^{2} + bx + c)}}{(x - \alpha)^{2}}$
$= \lim_{x \rightarrow \alpha}\frac{\sin{a(x - \alpha)^{2}}}{a(x - \alpha)^{2}} \times a$
$= \lim_{x \rightarrow \alpha}{a(1) =}a$