Hi tanvis1120,

The argument in this passage is related to a plan, specifically that the government's plan to produce information cards will result in angler's reporting any ruffe they catch (so the government can track the ruffe's spread).

The assumption is what must be true for the government's plan to work: that anglers will contact the government when they catch a ruffe (i.e. if anglers DO NOT do this, the government won't be able to track the ruffe's spread). Let's look at our answers:

A. The ruffe has spiny fins that make it unattractive as prey.

This is irrelevant to the assumption. Eliminate A.

B. Ruffe generally feed at night, but most recreational fishing on the Great Lakes is done during daytime hours.

This actually weakens the assumption because it suggests that the ruffe would not be caught. Eliminate B.

C. Most people who fish recreationally on the Great Lakes are interested in the preservation of the lake whitefish because it is a highly prized game fish.

This answer strengthens the assumption that anglers will contact the government when they catch a ruffe, because it is in the best interest of anglers to do whatever will help preserve whitefish which are threatened by the ruffe.

D. The ruffe is one of several nonnative species in the Great Lakes whose existence threatens the survival of lake whitefish populations there.

This answer simply restates the premise in the main question stem. Eliminate D.

E. The bait that most people use when fishing for whitefish on the Great Lakes is not attractive to ruffe. Eliminate E.

The correct answer is C.

Most of the argument in CR consist of two kind of clauses:
1- Facts/Premise
2- Conclusion

Assumption is an unstated link between the fact and conclusion.

but in the given question, All three sentences are fact or premise.
The structure of CR is:
1st sentence: The Eurasian ruffe.. ecosystem (This sentence has its own fact & conclusion )
Type: Fact
2nd sentence: To help... the ruffle(describe Agencies' action)
Type: Fact
3rd sentence: The cards... they catch(describe Agencies' action)
Type: Fact

The conclusion is actually given in the question. The conclusion is agencies' action have intended effect.
you have to assume the conclusion true and then mark your answer.
so basically question is about strengthen the argument type.

I solved this question by POE(Process of elimination) method and got the answer as C.