(A) tells us that few serious ecological problems are the result of government mismanagement. In other words, most serious ecological problems are not the result of government mismanagement. This is exactly the bridge we want. Once we know that most serious ecological problems aren't caused by government mismanagement, they get funneled into the conditional chain:

(We're meant to understand "ecological problems" and "environmental problems" as referring to the same thing. The difference in wording doesn't matter.)

(B) tells us about environmental problems that do stem from government mismanagement. But the conditional chain applies only to problems that are not from government mismanagement. So (B) is talking about the wrong category of problems entirely, and it doesn't establish anything about serious ecological problems. Without connecting serious ecological problems to the chain, we can't reach the conclusion.

(C) tells us that major changes in consumer habits can be made economically enticing. That's nice, but it doesn't establish anything about serious ecological problems. The gap in this argument isn't about whether consumer habit changes can be made enticing. The gap is about whether serious ecological problems are the kind of problems that require those changes in the first place. (C) leaves that gap wide open.

(D) goes in the wrong direction. We need to know that most serious ecological problems are not caused by government mismanagement. (D) tells us that most problems not caused by government mismanagement are serious ecological problems. That's "most As are Bs," not "most Bs are As."

To see the difference: imagine 100 problems aren't caused by government mismanagement, and 90 of those are serious ecological problems. (D) is satisfied. But there could be 1,000 serious ecological problems total, with 910 of them caused by government mismanagement. Now the vast majority of serious ecological problems don't trigger the conditional chain in the premises, and we can't reach the conclusion.

(E) tells us that few serious ecological problems can be solved by major changes in consumer habits. But knowing that consumer habit changes won't solve most serious ecological problems doesn't tell us what is required to solve them. Maybe most of them require government action, and government action doesn't need to be economically enticing. (E) gives us no basis for concluding that economically enticing solutions are necessary.