All signs need to have a _______.

A solid foundation is essential for all signs because it ensures stability and durability, which are crucial for visibility and safety. Signs are often exposed to various environmental conditions, including wind, rain, and snow, which can put stress on them. A solid foundation helps to anchor the sign securely in place, preventing it from tipping over or getting damaged. This is particularly important for permanent signage or those placed in high-traffic areas where they may be subject to impact or physical disturbance.

The other options focus on features that might be beneficial for specific types of signs but do not universally apply to all signs. For instance, a removable footing or retractable case could be useful for portable signs but do not provide the steadfastness that a solid foundation does. A flexible connection may offer some advantages in terms of movement or flexibility, but it could also compromise stability. Therefore, establishing a solid foundation is the fundamental requirement for ensuring that signs remain effective and secure over time.