According to 3.3.1.2 c) of ICAO Annex 2, a flight plan is required for "any flight across international borders", however there is no such thing as an "IFR boundary"; hence, answer a) is wrong.
Answer c) is wrong because a flight plan must be filed for "any flight or portion thereof to be provided with air traffic control service" (3.3.1.2 a) of ICAO Annex 2), not for any flight within controlled airspace. Think of VFR flights in airspace class E - controlled airspace, but no clearance or flight plan required.
Last edited by hvogt; 25th July 2013 at 06:55.