Lời giải

\( f'(x) = -e^x f^2(x)\Leftrightarrow  \frac{f'(x)}{f^2(x)} = -e^x \)  

\(\Rightarrow \int \frac{f'(x)}{f^2(x)} \, dx = \int -e^x \, dx  \Rightarrow -\frac{1}{f(x)} = -e^x + C \)  

\( \Leftrightarrow f(x) = \frac{1}{e^x + C}. \)  

 \( f(0) = \frac{1}{2} \Leftrightarrow \frac{1}{1 + C} = \frac{1}{2} \Rightarrow C = 1\)  

\( f(x) = \frac{1}{e^x + 1} \Rightarrow f(\ln 2) = \frac{1}{3} \Rightarrow \boxed{C} \)