•what environmental issues has the u. s. been struggling to address ever since 1970

Since the 1970s, the United States has struggled with two important environmental issues:

1. Oil and Fossil Fuels: After the oil crisis of 1973, the United States understood that it cannot solely rely on imported oil. Increased drilling in the US was required but this was also going to create problems for the environment.
Offshore drilling has been a major component of this concern and different administrations have been again or for increase drilling citing either environment or national security concerns. 

2. Nuclear: The cold war saw the US and Russia on the brink of nuclear war. During the 70s, the general American public became increasingly aware of the dangerous of just not Nuclear Weapons but also peaceful nuclear power and waste problems.

The answer should be b.