(c)
$f(x) = \frac{\sqrt{a^{2} - ax + x^{2}} - \sqrt{a^{2} + ax + x^{2}}}{\sqrt{a + x} - \sqrt{a - x}} \times \frac{\sqrt{a^{2} - ax + x^{2}} + \sqrt{a^{2} + ax + x^{2}}}{\sqrt{a^{2} - ax + x^{2}} + \sqrt{a^{2} + ax + x^{2}}} \times \frac{\sqrt{a + x} + \sqrt{a - x}}{\sqrt{a + x} + \sqrt{a - x}}$
$\Rightarrow \lim_{x \rightarrow 0}\ f(x)$
$= \lim_{x \rightarrow 0}\ \frac{- 2ax(\sqrt{a + x} + \sqrt{a - x})}{2x\ (\sqrt{a^{2} - ax + x^{2}} + \sqrt{a^{2} + ax + x^{2})}}$
$= \frac{- a(2\sqrt{a})}{(a + a)} = - \sqrt{a}$