Answer
The atom of mass $m$ will oscillate back and forth about the point $r = 0.2~nm$ where $U$ is a minimum.
Work Step by Step
Since the total energy in the system is less than zero, there is not enough energy for the atoms to escape from each other.
Since $m \lt \lt M$, then the atom of mass $m$ will oscillate back and forth about the point $r = 0.2~nm$ where $U$ is a minimum.
The leftmost point reached by the atom of mass $m$ will be the point $r \lt 0.2~nm$ where the energy $E$ is equal to the potential energy $U$
The rightmost point reached by the atom of mass $m$ will be the point $r \gt 0.2~nm$ where the energy $E$ is equal to the potential energy $U$
