Which of the following is considered a fire-resistive wall or floor?

A firewall is considered a fire-resistive wall because it is specifically designed to withstand the spread of fire and limit its effects on adjacent structures. Firewalls are constructed using non-combustible materials and are intended to separate different occupancy areas or sections of a building to prevent fire from traveling between them. They are usually built extending from the foundation through the roof and are often tested and rated for their fire resistance, which is measured in hours.

In contrast, options such as riser systems, smoke shields, and heat barriers serve different functions related to fire safety. Riser systems are vertical shafts that house plumbing and electrical systems, but they are not specifically designed for fire resistance. Smoke shields are barriers that help control smoke movement but do not necessarily provide the same fire-resistive capabilities as a firewall. Heat barriers can reduce heat transfer but lack the comprehensive fire-resistive properties that characterize firewalls. Therefore, the firewall stands out as the correct answer for identifying a true fire-resistive construction element.