$\lim\limits_{\alpha \rightarrow \pi / 4} \frac{\sin \alpha-\cos \alpha}{\alpha-\frac{\pi}{4}}=$

$\sqrt{2}$

$\lim\limits_{\alpha \rightarrow \pi / 4} \frac{\sin \alpha-\cos \alpha}{\alpha-\pi / 4}$

Using L'Hopital's Rule

so $\lim\limits_{\alpha \rightarrow \pi / 4} \frac{\cos \alpha+\sin \alpha}{1}=\frac{2}{\sqrt{2}}=\sqrt{2}$

Hence (3) is the correct answer.