I've been studying a section regarding flight plans, I've learned a fair bit about the ICAO flight plans so far, but I'm having trouble/confusion with a sample question in one of my study guides.

For a flight between Canada and Mexico:

an ICAO flight plan must be filed for all flights?
an ICAO flight plan must be filed only on a commercial flight?
an ICAO flight plan must be filed only if the flight will be in high level airspace?
or a transport canada flight plan will meet the requirements?

Does anybody have some info on this or a clear answer? Would really appriciate it! thanks!