What is the maximum value of the function $\sin \: x + \cos x$?

Question

$\begin{array}{1 1} \text{maximum value of}f(x)=\sqrt 2 \\ \text{maximum value of} f(x)= 0 \\\text{minimum value of} f(x)=\sqrt 2 \\ \text{maximum value of }f(x)= -\sqrt 2 \end{array}$