40 CFR Part 53 is primarily concerned with Ambient Air Monitoring.

• A. 40 CFR Part 50 establishes National Ambient Air Quality Standards and related limits.
• B. 40 CFR Part 60 covers standards for new stationary sources.
• C. 40 CFR Part 61 governs asbestos and other pollutants.
• D. 40 CFR Part 53 addresses Ambient Air Monitoring.

Answer: (D)

The main idea is to know which CFR part governs how ambient air data are collected, managed, and verified. 40 CFR Part 53 specifically sets the requirements for ambient air monitoring programs, including where monitors should be located, how they should be operated, and how data quality is assured. This covers the practical side of measuring outdoor air quality and ensuring the measurements are reliable for evaluating compliance with air-quality standards.

In contrast, Part 50 deals with National Ambient Air Quality Standards and the corresponding measurement methods and data interpretation. Part 60 focuses on standards for new stationary sources, and Part 61 addresses asbestos and other specific pollutants. Those parts relate to setting limits or controlling emissions rather than the ongoing operation and QA of ambient monitoring networks.