The correct option is D The force of interaction between the particles is repulsive between points E and F.
At B and D, dUdr≠0, so they are not equilibrium points.
At C, dUdr=0 and potential energy is minimum. So, it is a stable equilibrium position.
Now, F=−dUdr= − (Slope of U−r curve)
In the region CD, the slope of the graph is positive, therefore by the relation, F is negative i.e, force is attractive in nature.
In the region DE, the slope of the graph is again positive, therefore by the relation, F is negative i.e, force is attractive in nature in the region DE too.
But in the region EF, the slope of the graph is negative which means F is positive i.e, force is repulsive in nature.