-------- legally approved the establishment of National ambient air quality standardsa.CAA 1980b.CAA 1950c.CAA 1960d.CAA 1970

To answer the question regarding which CAA (Clean Air Act) legally approved the establishment of National Ambient Air Quality Standards (NAAQS), we need to consider the timeline and provisions of each mentioned amendment.

1. Overview of the Clean Air Act:

   • The Clean Air Act (CAA) was originally passed in 1963, but it underwent significant amendments in subsequent years.

2. Key Amendments:

   • CAA 1970: This amendment significantly expanded the federal government's authority to regulate air pollution. It established the National Ambient Air Quality Standards (NAAQS), providing the framework for setting standards to protect public health and the environment.
   • CAA 1980 and 1990 Amendments: These provided further modifications and updates but were not responsible for the original establishment of NAAQS.

3. Conclusion:

   • Given the historical context and the importance of the CAA 1970 in establishing air quality standards, we can conclude the answer.

Final Answer

The correct answer is d. CAA 1970.