Which type of fire doors must secure the door when closed with an active latch bolt?

The requirement for both single fire doors and pairs of fire doors to secure properly when closed with an active latch bolt is rooted in fire safety standards. Fire doors are crucial in containing a fire and preventing its spread while providing safe egress paths.

When a fire door is installed, it must be equipped with a latch bolt mechanism that ensures the door is fully secured when closed. This mechanism allows the door to close snugly against the frame, minimizing any gaps that could allow smoke or flames to pass through. For single fire doors, the latch bolt serves the same purpose, ensuring that the door remains shut under heat or pressure conditions established during a fire.

Similarly, for a pair of fire doors, the latch bolt system is essential as well. Both doors in a pair must be able to seal adequately to prevent smoke and flames from moving throughout a building. When both doors are closed, they should latch securely to maintain their integrity as a fire barrier.

Therefore, recognizing that both configurations of fire doors must meet the same requirement regarding the latch bolt establishes that the correct answer encompasses both single fire doors and pairs of fire doors.