Which of the following requirements must be met for a duct to penetrate three floors without a fire damper at each floor?

For a duct to be permitted to penetrate three floors without the installation of a fire damper at each floor, it must typically be contained within a wall cavity. This requirement is important because enclosing the duct within a wall cavity helps to prevent fire and smoke from spreading through the duct system. The wall cavity acts as a barrier, helping to maintain fire separations that are crucial for safety in multi-story buildings.

In addition to being contained within a wall cavity, other factors such as the type of material used for the duct, its dimensions, and its configurations may influence compliance with fire codes. However, if the duct is not within a wall cavity, fire dampers would generally be required at each floor to mitigate risks associated with fire and smoke movement through the ductwork. Thus, the correct choice emphasizes the necessity of the wall cavity for enhancing fire safety in this context.