It is better for the environment if as much of all packaging as possib

Def. A. If I were short on time, i wouldn't bother w/ the other answers b/c A is such a great answer.

However, if u do have time, never just pick A and move on.

B: This somewhat weakens the argument. It says that SOME plastic is biodegradable. Suggesting that changing to plastic would not be 100% horrible for the environment. But this is a very small weaken. It doesn't really explain the remaining plastics. What if all the paper and all the cardboard were biodegradable? Then changing to plastics would not help the environment. Wed need more info to support this answer choice.

C.Irrelevant.
D. So? Paper and Cardboard could still be much better than plastics. This choice doesn't address this.
E. Irrelevant, doesn't weaken the argument in the least.

(A) Correct. If paper and cardboard are not biodegradable then they are not better. This statement undermines the author's conclusion significantly.
(B) Does nothing to undermine the conclusion.
(C) Space in landfills is not an issue here. Eliminated.
(D) Impossible is an extreme word here. Eliminated.
(E) Plastic, whether adulterated or not, it is still not biodegradable

In life? As in, "isn't it just an actual fact that paper and cardboard used in packaging are biodegradable?"

I suspect it is. But that doesn't mean it wouldn't weaken the argument IF TRUE (notice that phrase in the question!)

In Critical Reasoning, you are dealing with the REASONING in the argument, not the TRUTH of it. Assumptions have two features: They are not stated, and B, they must be true for the argument to hold.

"I should move to Healthtown. Their healthcare system is really good, so I'll probably live a longer life there than I would in my current home of Diseasesville."

Would it weaken the argument if it were true that "Healthtown is completely devoid of oxygen"? Absolutely it would. The argument is assuming many things, but one of them is that Healthtown has all sorts of the necessary components that 'life' requires, including oxygen. Now, it's a VERY SAFE assumption that Healthtown has plenty of Oxygen... But it's still an assumption, and so, yes, it would weaken the argument IF IT WERE TRUE that "Healthtown is devoid of oxygen."

if possible please explain me the last sentence that is "therefore, it is always a change for a worse to replace packaging material made from paper or cardboard with packaging material made from plastics"

here the assumption is 'packaging materials from Paper are bio-degradable' to arrive at the conclusion. the option A attacks this assumption.

Need an expert's explanation on this.
Isn't the answer opposing an actual fact?

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.