Explanation
Water Efficiency is the only credit category that does not have a prerequisite in LEED v4 for Building Design and Construction. Prerequisites are the minimum requirements that all projects must meet to be eligible for LEED certification. They are mandatory and non-weighted, meaning they do not contribute to the project's score. The other credit categories have at least one prerequisite each, as shown in the table below:

References: LEED v4 Reference Guide for Building Design and Construction, p. 17 1; [LEED v4 BD+C Scorecard]

Explanation
Building-Level Water Metering is a prerequisite in LEED v4 Water Efficiency category. Building-Level Water Metering means that the project must install permanent water meters that measure the total potable water use for the building and associated grounds. The project must also commit to sharing the water usage data with USGBC for at least five years after occupancy or certification. The LEED Green Associate Candidate Handbook states that this is one of the two prerequisites in the Water Efficiency category [1, p.
14]. References: [LEED Green Associate Candidate Handbook], [Building-Level Water Metering | U.S. Green Building Council]

Explanation
Light, sound, occupant experience are some of the components that comprise the Indoor Environmental Quality credit category. The Indoor Environmental Quality credit category aims to provide a healthy, comfortable, and productive indoor environment for building occupants by addressing various factors that affect their well-being, such as thermal comfort, lighting quality, acoustic quality, air quality, and occupant control. The LEED Green Associate Candidate Handbook states that one of the intents of the Indoor Environmental Quality category is to "enhance indoor environmental quality (IEQ) by improving the design, construction and operation of buildings" [1, p. 16]. References: LEED Green Associate Candidate Handbook,
[Indoor Environmental Quality | U.S. Green Building Council]