Which type of fire door is often found in old industrial buildings and slides into position by gravity?

The correct choice is overhead rolling steel fire doors. These doors are specifically designed for industrial applications and are often installed in older industrial buildings due to their durability and effectiveness in fire prevention.

Overhead rolling steel fire doors operate by rolling up and down along tracks, and when not in use, they are usually stored in a rolled position at the top of the opening. During a fire, these doors can be designed to close automatically, often relying on gravity or a mechanical system to descend into place, effectively creating a barrier that can contain fire and smoke, helping to protect other areas of the building. This design is particularly suited for larger openings typically found in industrial contexts.

Other options like horizontal sliding fire doors and vertical sliding fire doors serve different functions and are less commonly associated with the gravity-operated closing mechanism highlighted in the question. Swinging fire doors operate on hinges and are generally used in commercial and residential settings rather than in industrial buildings. Thus, the characteristics of the overhead rolling steel fire door align closely with the scenarios presented in the question.