All laundry detergents contain surfactants

To be clear, D) is wrong because it contradicts the premise correct? That's a no-no on the GMAT.

All laundry detergents contain surfactants, which can harm aquatic life. However, the environmental effects of most ingredients in laundry detergents, including most of those in so-called "ecologically friendly" detergents, are unknown. Therefore, there is no reason to suppose that laundry detergents advertised as ecologically friendly are less damaging to the environment than other laundry detergents are.

Which one of the following, if true, most weakens the argument?

(A) Laundry detergents that are advertised as ecologically friendly contain much lower amounts of surfactants, on average, than do other laundry detergents. CORRECT
(B) There is no reason to suppose that most of the ingredients in laundry detergents not advertised as ecologically friendly harm the environment significantly. X
(C) Different kinds of laundry detergents contain different kinds of surfactants, which differ in the degree to which tlhey could potentially harm aquatic life. X
(D) There is reason to suppose that ingredients in laundry detergents other than surfactants harm the environment more than surfactants do. X
(E) Laundry detergents advertised as environmentally friendly are typically less effective than other detergents, so that larger amounts must be used. X

D is incorrect because the comparison between dangerousness of "surfactants" and "other ingredients" does not matter as we do not know the ingredients of "ecologically friendly" Or other detergents

All laundry detergents contain surfactants, which can harm aquatic life. However, the environmental effects of most ingredients in laundry detergents, including most of those in so-called "ecologically friendly" detergents, are unknown. Therefore, there is no reason to suppose that laundry detergents advertised as ecologically friendly are less damaging to the environment than other laundry detergents are.

Which one of the following, if true, most weakens the argument?

(A) Laundry detergents that are advertised as ecologically friendly contain much lower amounts of surfactants, on average, than do other laundry detergents.
-> This gives us reason to see why the ecologically friendly detergents could be less damaging. This is the right weakener

(B) There is no reason to suppose that most of the ingredients in laundry detergents not advertised as ecologically friendly harm the environment significantly.
-> Okay but these detergents could still be little more harmful / equally harmfully / less harmful . Theres just no reason to see them as SIGNIFICANTLY harmful.
So this has no impact on the argument

(C) Different kinds of laundry detergents contain different kinds of surfactants, which differ in the degree to which they could potentially harm aquatic life.
okay so do the ecologically friendly detergents have which kind of surfactant ? one that does more harm or less?
again no impact on the argument

(D) There is reason to suppose that ingredients in laundry detergents other than surfactants harm the environment more than surfactants do.
- okay good, so these kind of ingredients are there in both the types of detergents ? if not then is there more harmful ingredients in the ecological friendly one or not ?
again no impact with this option.

(E) Laundry detergents advertised as environmentally friendly are typically less effective than other detergents, so that larger amounts must be used.
- Does larger amount mean more harm ? it could very well be that the ingredients in the environmentally friendly detergent are just less harmful regardless of the amount. Or they could be more harmful. knowing the Quantity doesn't help here.

All laundry detergents contain surfactants, which can harm aquatic life. However, the environmental effects of most ingredients in laundry detergents, including most of those in so-called "ecologically friendly" detergents, are unknown. Therefore, there is no reason to suppose that laundry detergents advertised as ecologically friendly are less damaging to the environment than other laundry detergents are.

Which one of the following, if true, most weakens the argument?

(A) Laundry detergents that are advertised as ecologically friendly contain much lower amounts of surfactants, on average, than do other laundry detergents. - CORRECT. We have a reason to believe that ecologically friendly ones are less damaging to environment.
(B) There is no reason to suppose that most of the ingredients in laundry detergents not advertised as ecologically friendly harm the environment significantly. - WRONG. Harming whether significantly or not is not the question. Irrelevant.
(C) Different kinds of laundry detergents contain different kinds of surfactants, which differ in the degree to which they could potentially harm aquatic life. - WRONG. Again an open scenario is presented. So, nothing is certain.
(D) There is reason to suppose that ingredients in laundry detergents other than surfactants harm the environment more than surfactants do. - WRONG. Irrelevant. The effects are not known as said in passage.
(E) Laundry detergents advertised as environmentally friendly are typically less effective than other detergents, so that larger amounts must be used. - WRONG. Even if large amounts is used, we are left with a question as in how much is that amount. It remains a open possibility wherein amount is smaller than non-ecologically friendly detergents or larger.

Answer A.