The commissioner defends the position by saying that these fishes are indigenous to the mountain ponds and we need to defend any fish that belongs here.
To weaken the commissioner's position we need to say that these fishes do not belong here.

(A) The suckers, shiners, and chubs in mountain ponds are descendants of fish brought to those ponds by anglers for use as bait in catching the once abundant trout.
Correct.

(B) The state's policy of preserving natural aquatic ecosystems favors the interests and hobbies of one group of people over those of other groups.
interests and hobbies are not mentioned.

(C) Suckers, shiners, and chubs are dull, unattractive fish with none of the graceful beauty of trout.
Aesthetic values of fishes are not important.

(D) People who fish in mountain ponds have a deeper appreciation of nature than do people who never visit such areas.
Out of scope.

(E) A large percentage of the fish in mountain ponds could be eliminated and replaced with trout without seriously disturbing any plants or animals in the surrounding woodlands.
disturbance of plants or animals in the surrounding woodlands is not mentioned.

The explanation is collected from a GMATclub member because I didn't understand it from here.

The commissioner's statement rests on the proposition that any indigenous species should be preserved so as to not harm the existing acquatic ecosystems. To weaken the statement of fish and game commissioner, we have to show that the species mentioned by Angler should not be indigenous species ( native to the land ) i.e. it should be either implanted or bred there for some other purpose.

(A) The suckers, shiners, and chubs in mountain ponds are descendants of fish brought to those ponds by anglers for use as bait in catching the once abundant trout. Option A exactly does weaken the propostion of the commissioner. Hence keep the option.
(B) The state's policy of preserving natural aquatic ecosystems favors the interests and hobbies of one group of people over those of other groups. Out of scope

(C) Suckers, shiners, and chubs are dull, unattractive fish with none of the graceful beauty of trout. Out of scope since beauty is not the determinig factor

(D) People who fish in mountain ponds have a deeper appreciation of nature than do people who never visit such areas.Again out of scope since appreciation of the nature by angler is not the determining factor for preserving the species

(E) A large percentage of the fish in mountain ponds could be eliminated and replaced with trout without seriously disturbing any plants or animals in the surrounding woodlands. Only slighlty weakens since lets some XYZ research paper says that some of the species mentioned by Angular has been native to the pond since pre-historic times, then commissioner has violated the state policy

Since rest options is eliminated, answer is IMO A

If suckers, shiners and chubs are descendants of fish that were only brought to b found as bait then the commissioner cannot say that they are Indigenous to the area. Hence through option a , commission's argument is weakened.

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