(d)
Given, $\lim_{x \rightarrow 0}\frac{\left\{ (a - n)nx - \tan x \right\}\sin{nx}}{x^{2}} = 0$
$\Rightarrow \ \ \ \ \ \ \lim_{x \rightarrow 0}{\left( (a - n)n - \frac{\tan x}{x} \right).}\frac{\sin{nx}}{x} = 0$
$\Rightarrow \ \ \ \ \ \ \ \ \left\lbrack \left\{ a - n \right\} n - 1 \right\rbrack n = 0$
$\Rightarrow \ \ \ \ \ \ \ \ \ a = n + \frac{1}{n}$