詳解：\(y = \sqrt x \) ，對 \(x\) 微分，得

\(\frac{{dy}}{{dx}} = \frac{{d\left( {\sqrt x } \right)}}{{dx}} = \frac{1}{{2\sqrt x }}\) ，令 \(x = \frac{1}{4}\) 表示在 \(x = \frac{1}{4}\) 時之瞬間變化率

\(\frac{dy}{dx} = \frac {1}{2\sqrt \frac{1}{4}} =1\)

故選(A)