Which systems refer to mechanical systems installed prior to code adoption?

The choice of "Existing Mechanical Systems" is accurate because it specifically refers to mechanical systems that were in place before the implementation of new building codes. When mechanical systems are installed prior to the adoption of current regulations, they are categorized as existing because they are already operational and may not necessarily comply with current code requirements.

The term "existing" effectively captures the status of these systems as they are distinguished from new installations that must conform to updated codes. These existing systems might remain in use even as codes evolve, which highlights the need for inspections and evaluations to ensure safety and efficiency when changes are made or systems are upgraded.

In contrast, the other options imply different states or conditions of mechanical systems. Systems described as retrofitted often involve modifications or improvements to bring them up to current standards, while upgraded systems suggest enhancements or replacements intended to enhance performance or compliance. Modernized systems typically refer to those that have incorporated the latest technology or efficiency standards, which would not apply to systems installed prior to code adoption. Thus, "Existing Mechanical Systems" is the most fitting description for those systems operational before new regulations took effect.