Understanding the argument -
It is better for the environment if as much of all packaging as possible is made from materials that are biodegradable in landfills. - Fact. Say 99% of the packaging is made with biodegradable materials is ENOUGH or SUFFICIENT condition for a better environment. Biodegradable packaging is a 100% guarantee for a better environment.
Therefore, it is always a change for the worse to replace packaging made from paper or cardboard with packaging made from plastics that are not biodegradable in landfills. - Conclusion. It's a change for the worse. What change? Packaging with paper or cardboard to packaging with plastics. Why it's worse? Because "paper or cardboard" must be better than "plastics"? We are moving from good to bad? Yes. That's why it's worse.
Option Elimination - Weakener
(A) The paper and cardboard used in packaging are usually not biodegradable in landfills. - Oh. So, they are not biodegradable. So we have
1. Paper and cardboard are non-biodegradable.
2. Plastics - non-biodegradable.
So how can a shift from 1 to 2 is worse? 1 & 2 are both worse. Ok.
(B) Some plastic used in packaging is biodegradable in landfills. - some can be, say, 2 or 2%. But the rest 98% is still non-biodegradable. So, the conclusion still holds? Yes. That is not our answer. Distortion.
(C) In many landfills, a significant proportion of space is taken up by materials other than discarded packaging materials. - "Materials other than discarded packaging materials" is out of scope.
(D) It is impossible to avoid entirely the use of packaging materials that are not biodegradable in landfills. - whether it's possible or impossible is out of scope. For now, our scope is to weaken the conclusion: "It is always a change for the worse."
(E) Sometimes, in packaging an item, plastics that are not biodegradable in landfills are combined with cardboard. - It essentially says that, say, 2% of the time, plastics are used with cardboard. So, the usage of plastics is unavoidable 2% of the time. But how about 98% of the time? If the cardboard is replaced with plastics, can it still be a change for the worse? Yes, it can still be harmful, and the conclusion is valid. Unlike option A, this option doesn't say both are equally bad. Distortion.