1. The problem asks to find the interval where the derivative $f'$ of the function $f$ is negative. 2. From the graph description, $f$ has a maximum near $x=1$ and crosses the x-axis near $x=0$ and $x=2$. 3. The derivative $f'$ is negative where the function $f$ is decreasing. 4. Observing the graph, $f$ decreases between $x=1$ and $x=2$. 5. Therefore, $f'$ is negative on the interval $]1,2[$. Final answer: (a) $]1,2[$