A block rests on a rough floor while a horizontal force F is...

When the applied horizontal force $F$ is less than the limiting static friction, the block remains at rest. In this region, the static frictional force $f$ adjusts itself to be equal in magnitude to the applied force $F$. Thus, $f = F$, which represents a straight line passing through the origin with a slope of $45^{{}^{{}^{\circ}}}$.

When $F$ exceeds the limiting static friction $f_{s, max}$, the block begins to slide. The frictional force then becomes kinetic friction $f_{k}$. Since the coefficient of kinetic friction $\mu_{k}$ is generally slightly less than the coefficient of static friction $\mu_{s}$, the frictional force drops slightly from $f_{s, max}$ to $f_{k}$. This sudden drop appears as a small kink in the graph.

For all further increases in $F$ (large $F$), the kinetic friction remains constant at $f_{k}$, making the graph a straight line parallel to the $F$-axis.

Therefore, the graph has a slope of $45^{{}^{{}^{\circ}}}$ for small $F$, a small kink at the transition, and is parallel to the $F$-axis for large $F$.