Final answer:
One of the most pressing environmental issues prior to the 1990s was acid rain, caused by emissions of sulfur dioxide and nitrogen oxides from the burning of fossil fuels. Other significant issues during this time were ozone depletion and climate change.
Step-by-step explanation:
Prior to the 1990s, one of the most pressing environmental issues was (c) Acid rain. Acid rain is caused by emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) resulting from the burning of fossil fuels. It causes acidification of lakes and streams, damages trees and sensitive forest soils, corrodes metals, and contributes to human health problems. Other environmental issues that were also significant during this time include (b) Ozone depletion and (a) Climate change. Ozone depletion is caused by ozone-depleting substances (ODS) emitted into the atmosphere, while climate change is primarily driven by greenhouse gas emissions from the burning of fossil fuels.