The changes in gas-fired plants over time aren't relevant, because the only comparison we care about is between the current waste-to-energy and gas-fired options—and we already know the waste-to-energy plant loses on air pollution. This doesn't help justify the position that the waste-to-energy plant is still preferable.

Increasing energy use could explain why a new plant is needed, but that isn't relevant to which new plant we should build. If increased trash production had some environmental impact that could be offset by the waste-to-energy plant that might be helpful, but without that information, this just isn't relevant.

This describes an advantage to building the waste-to-energy plant, so it helps justify the environmentalists’ position. The waste-to-energy plant could reduce environmental harm by preventing another source of air pollution, so it's preferable to the gas-fired plant, which presumably would not replace the incinerator.

This does nothing to support the environmentalists’ position, because the waste-to-energy plant produces more air pollution than the gas-fired plant. We need an answer that helps justify their support for the waste-to-energy plant, but this doesn't add any advantages for that option.

The environmentalists are concerned with comparing the effects of two different power plant options; other sources of pollution are irrelevant to this comparison. This doesn't give us any reason why the waste-to-energy plant should be preferred, so it can't justify the environmentalists' position.