If you fly at cabin pressure altitudes above 10,000' for more than 30 minutes, each crew member must use supplemental oxygen. That means if you fly an unpressurized aircraft at 10,500' for 30 minutes, no oxygen is required, but if you fly at 10,500' for 31 minutes, 31 minutes of oxygen is required. This answer choice is not correct because you CAN legally fly above 10,000' without oxygen, as long as you don't go above 13,000', and as long as you don't stay there for more than 30 minutes.

Operating a US-registered aircraft in US airspace, you are required to have oxygen above 12,500'.

As described in CARs 605.31 and CARs 605.32, everyone on board is required to use supplemental oxygen when the cabin altitude is above 13,000'.