What must fences erected on a slope be in relation to the horizon?

Answer: B — Plumb; perpendicular to the horizon

Fences erected on a slope must be plumb, meaning they are perpendicular to the horizon. This orientation is crucial for several reasons. Firstly, having the fence plumb ensures its structural integrity and durability. If a fence is not installed perpendicularly, it may become unstable, especially under various weather conditions or load conditions such as wind, which could lead to damage or failure. Additionally, a plumb fence is more effective in providing safety and functionality. For instance, if the fence is being used to keep livestock contained or to protect a construction site, a plumb installation prevents gaps and ensures encroachment or escape is minimized. When considering the other options, while a fence could be angled or level with the slope, these orientations may lead to issues such as erosion or difficulty in maintenance, and they compromise the fence's capability to serve its intended purpose effectively. Installing the fence plumb allows for a consistent, reliable, and professional appearance while maximizing its longevity and performance.