Answer
${f_y}$ is smallest at $C$.
Work Step by Step
${f_y}$ can be approximated by ${f_y} \approx \frac{{\Delta f}}{{\Delta y}}$. This can be calculated by moving up vertically from points $A$, $B$, or $C$ to the next highest level curves. From Figure 8, we see that $\Delta y$ is largest at point $C$ (though $\Delta f$ is twice as much as those of $A$'s and $B$'s but $\Delta y$ is much larger), thus $\frac{{\Delta f}}{{\Delta y}}$ is smallest at $C$. We conclude that ${f_y}$ is smallest at $C$.
