1. **Stating the problem:** We need to find the value of $x$ where the rate of change of $f(x)$ is least. The rate of change corresponds to the slope of the tangent line to the curve $y=f(x)$, which is given by the derivative $f'(x)$. 2. **Understanding the problem:** The rate of change is least means the slope $f'(x)$ is at its minimum value. Since the graph is sinusoidal, the slope varies and the minimum slope is the steepest negative slope. 3. **Analyzing the graph description:** The slope is steep downward near $x = -1.3$, indicating the rate of change is least there. 4. **Conclusion:** The rate of change of $f(x)$ is least at $x = -1.3$. **Final answer:** (B) -1.3