Now I am not doing the ATPL theory so could get this wrong myself, however my idea would be that Bristol is correct.
If the maximum operating pressure of the oxygen system is exceeded the:
Passenger oxygen masks will drop down (the answer i gave)
Oxygen is discharged overboard via a safety plug. (bristol right answer)
safety plug to me could mean a safety releif valve!! which in turn means should the system be set up as 2 bar max operating pressure and then the system starts running over at say 3 bar (being over the max operating pressure) the system will vent the excess (blowout) using the safety plug.
Like i said i am not doing the ATPL's, so await to be corrected.
VFR