The lowest horizontal member of a truss is called a:

The term "bottom chord" refers specifically to the lowest horizontal member of a truss system. It serves a crucial structural role by connecting the ends of the diagonal members (webs) that make up the truss. This arrangement is essential for distributing loads and maintaining the stability and integrity of the truss. The bottom chord typically supports the weight of any roofing or flooring system above, making it an integral part of the overall truss design.

In contrast, other terms in the choices refer to different elements or functions within construction. For example, a brace is typically an inclined member that provides support to improve stability and prevent lateral movement in a structural frame. A ceiling joist forms part of the ceiling structure and is primarily used to support the ceiling materials, while a stay usually refers to a tension member used to support and stabilize structures, such as masts or towers. These members have distinct purposes and are not synonymous with the lowest horizontal member of a truss, reinforcing the correctness of identifying the bottom chord as the answer.