Good question - and you're just about entirely correct, which is what makes these "removes the flaw" / "assumption" types of questions so difficult.

If you first break down the question stem, it's saying that the (existing) conclusion (already) drawn in the argument needs one of answer choices to strengthen it.

It's convoluted wording, but it's an Assumption question. And with most difficult Assumption questions, the correct answer doesn't so much "strengthen" the conclusion as it removes a serious flaw in the conclusion. Here, if you take the argument as:

Premise: If the environmental problem isn't a government problem, it must be solved via consumer habits

Premise: Consumer habit changes will only occur if they're economically enticing.

Conclusion: Few serious environmental problems will be solved unless the solutions are economically enticing.


Well, what's the gap there? "Economically enticing" only relates to "consumer habits" and "consumer habits" are only necessary if it isn't a government problem.

What would weaken this argument? If most serious environmental problems were government problems, then the consumer habits wouldn't be necessary.

The right answer removes that flaw - by saying that "few serious ecological problems are government problems", it pre-empts the weakness that we found in the argument. It shows that most problems are the type that require a change in consumer behavior.

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Now, is the argument airtight even with A? No - like you said, we don't know how government-based problems are to be solved. But we do know that this argument is structured essentially like:

If the problem is not A, then B must happen.
B requires C.
So in order to solve the problem, we must have C.

If you strip down the logic to these variables, in order for the conclusion to hold true we need to either know:

This problem is A.
or
This problem is D, and D requires B, too.

Somewhere there's a gap - we only know the conclusion to be true if consumer habits must be changed. The most direct path to that conclusion is to show that most problems are of the type that we already know require consumer habit changes. And while choice (A) isn't the only way to do it, it's the only answer choice that does so.