I voted for "Separate Structure" and "Other". I haven't yet built my cinder block outbuilding but will soon. It'll be climate/humidity controlled and finished inside with a false wall behind which valuables will be hidden. The "other" part is that all firearms (those not kept in the house for SD/HD) will be stored in USGI 12"x12"x48" steel sealed containers with desiccant (crystal kitty litter)... containers behind the false wall. A security system will be employed.
I opted for this because it offers better protection for both fire and theft... "out-of-sight-out-of-mind" is far better than any safe and a "fireproof" outbuilding is far better than any "fire resistant" gun safe.
What will make your building "fireproof"?