No person shall fly an unpressurized aircraft for more than . . . . . at an altitude between 10,000 and 13,000 feet ASL unless there is readily available to each flight crew member, an oxygen mask and a supply of oxygen.
Why this is the answer
Per CAR 605.31, between 10,000 and 13,000 ft ASL an unpressurized aircraft may be flown for at most 30 minutes before crew oxygen must be readily available. The shorter and longer intervals are traps.
Reference
CARs 605.31 – Oxygen Equipment and Supply
Question reproduced verbatim from Transport Canada’s PSTAR Study and Reference Guide (TP 11919E). The answer key is Transport Canada’s; the explanation is ours.
A study aid — not a substitute for the official guide, the CARs, or your instructor.