Which of the following constructions must be supported by concrete footings directly in contact with the earth?

Permanently installed structures must be supported by concrete footings directly in contact with the earth because they are intended for long-term use and stability. Concrete footings provide the necessary strength and support to carry the weight of the structure and distribute it evenly across the ground. This is critical for preventing settling or shifting over time, which could compromise the integrity of the building. The footings not only anchor the structure securely but also help to prevent movement due to environmental factors such as frost heave or soil erosion.

In contrast, temporary structures are not designed for long-term stability and may not require the same type of foundational support. Similarly, modular structures may be designed for less permanence and can sometimes be supported on alternative systems. Parking structures, while also requiring a sturdy foundation, fall under the category of permanently installed structures and would typically require concrete footings. However, the focus here is specifically on structures intended for permanent installation and their need for solid grounding, making the identification of permanently installed structures as the correct response clear.