1. The problem asks to find the interval where the derivative $f'$ of the function $f$ is negative. 2. The derivative $f'$ is negative where the function $f$ is decreasing. 3. From the graph description, $f$ rises from $x=0$ to $x=1$, reaches a peak at $x=1$, then decreases from $x=1$ to $x=2$, crossing the x-axis at $x=2$, and then rises again after $x=2$. 4. Therefore, $f'$ is negative on the interval where $f$ is decreasing, which is between $x=1$ and $x=2$. 5. The interval notation for this is $]1,2[$. 6. Among the options given, (a) $]1,2[$ matches this interval. Final answer: (a) $]1,2[$