What major environmental agreement aimed at combating climate change was signed in 1997?

The Kyoto Protocol is the correct choice because it was the landmark international agreement established in 1997 that specifically aimed at reducing greenhouse gas emissions to combat climate change. It was negotiated in Kyoto, Japan, and represented a commitment from participating industrialized countries to reduce their emissions of carbon dioxide and other greenhouse gases. This agreement marked a significant step in international efforts to address climate change, emphasizing the responsibilities of developed nations in the global environmental crisis, and provided a framework for future agreements on climate action.

The other agreements and acts mentioned have different focuses or purposes. For instance, the Paris Agreement, though also a significant climate accord, was established later in 2015. The Clean Air Act is a major piece of legislation aimed at controlling air pollution on a national level, but it does not specifically target climate change in the same way as the Kyoto Protocol. The Montreal Protocol, signed in 1987, was crucial in addressing ozone-depleting substances but does not specifically deal with climate change like the Kyoto Protocol does. Therefore, the focus of the Kyoto Protocol on climate change makes it the correct answer to the question asked.