The incorrect statement is Option C. - At very short separations ( r<0.8‌fm ) the nuclear force is strongly repulsive (a "hard core"). - For distances r≃0.8−3‌fm it becomes attractive. - The potential energy reaches its minimum (stable equilibrium) at roughly r=0.8‌fm. Since the force is not always attractive (it's repulsive at r<0.8‌fm ), saying "nuclear force is always attractive" is wrong.