According to the water efficiency category, what prerequisite applies to all projects?

The correct answer is that Building Level Water Metering is the prerequisite that applies to all projects according to the water efficiency category in LEED v4. This prerequisite requires that all projects implement a metering system for water use at the building level to monitor actual water consumption. This enables project teams to identify water usage patterns, implement conservation measures, and track efficiency over time.

Having building-level metering helps to establish a baseline for water use, which is essential for ongoing management strategies that promote water efficiency. This data is critical for understanding how water is consumed within the building and for establishing goals for reduction in water use. By focusing on building-level water metering, LEED encourages a proactive approach to managing water resources, contributing towards a more sustainable building design.

The other answer choices pertain to additional options that can contribute to water efficiency in projects but are not prerequisites applicable to all projects. Indoor Water Use Reduction focuses on minimizing water consumption within the building, but it is not required for all projects at the prerequisite level. Cooling Tower Water Use and Advanced Water Metering are also strategies that can be implemented for credits but are not universal prerequisites.