Fire-rated assembly designed to prevent fire spread by auto sealing an opening in a fire wall to block spread?

• A. Fire Stop
• B. Fire Door
• C. Fire Wall
• D. Cast-in-place Concrete

Answer: (B)

A fire door is the assembly that fits into a fire-rated wall opening and can automatically seal the opening to stop fire and smoke from passing through. When exposed to heat, intumescent seals around the door edge (often with a self-closing device) expand and the gap around the door is sealed, creating a barrier in the wall. This is exactly how it blocks spread through that opening.

The other options don’t fit the scenario: a fire stop seals penetrations like pipes or cables but does not close a wall opening with a door; a fire wall is the wall itself, not the sealing device; cast-in-place concrete is a material, not an opening-sealing door assembly.