It's very important to firstly differentiate 'immigration' from 'invasion'. The two are not the same thing. Invasion is usually characterized by the takeover of a country or territory by force, through violent methods. Immigration on the other hand, occurs when one or more person tries to enter a territory/country that is not its native one peacefully, with no intention of taking it over, but simply to make a life on this new territory. e.g. If a professional receives a proposition to work abroad, in a country that is not its native one (France, per say), and moves with his or her family there, this process may be characterized as an immigration (but never as an invasion).
Those being said, you could immediately discard letters B and D.
In letter C, if the soldiers were fighting their way out of an ambush, you can assume that soldiers from both sides were at the same territory at the moment, therefore no invasion process was in course.
Finally, the answer is letter A. Although no explicit warlike army invasion occured, the spy would try to bring the U.S. government down by force (not in a democratic method, and that could already characterize an invasion).
