Answer
The force is repulsive when $~~0 \lt r \lt 0.2~nm$
Work Step by Step
The force $F$ is repulsive when $F \gt 0$
We can write an expression for $F$:
$F = -\frac{dU}{dx}$
For the values of $r$ such that $0 \lt r \lt 0.2~nm$, the slope of the $U$ versus $r$ curve is negative. Then the value of $\frac{dU}{dx}$ is negative, and the force $F$ is positive.
The force is repulsive when $~~0 \lt r \lt 0.2~nm$
