$\int \frac{d x}{\sqrt{a^{2}-x^{2}}}=?$

$\sin ^{-1} \frac{x}{a}+c$

$\cos ^{-1} \frac{x}{a}+c$

$\frac{1}{a} \sin ^{-1} \frac{x}{a}+c$

$\frac{1}{a} \cos ^{-1} \frac{x}{a}+c$