$$y=\sqrt{1+\sin x}$$

(正答率 82%)

$$\begin{eqnarray*} y'&=& \frac{1}{2}(1+\sin x)^{-\frac{1}{2}}\times(1+\sin x)'\\ &=& \frac{\cos x}{2\sqrt{1+\sin x}} \end{eqnarray*}$$