(A) establishes that most serious ecological problems are NOT the result of government mismanagement. (We’re meant to understand that ecological problems are by definition environmental problems, so the fact (A) doesn’t say “environmental” doesn’t matter.) This triggers the conditional chain in the premises. We can now conclude that, for most serious ecological problems, their solutions require a change in consumer habits, which in turn requires making the solution economically enticing.

(B) doesn’t establish anything about “serious ecological problems.” Since neither this answer nor the premises establish anything about “serious ecological problems,” it cannot make the argument valid.

(C) doesn’t establish anything about “serious ecological problems.” Since neither this answer nor the premises establish anything about “serious ecological problems,” it cannot make the argument valid.

(D) reverses what we want. We want to prove that most serious ecological problems are not the result of government mismanagement. (D) tells us that most problems that aren’t the result of government mismanagement are serious ecological problems. But this leaves open the possibility that only a very small proportion of serious ecological problems fall into the category of “not a result of government mismanagement.” (”Most As are B” does not establish that “Most Bs are A.”)

(E) establishes that most serious ecological problems cannot be solved by major changes in consumer habits. But even if this is true, that doesn’t establish what is REQUIRED for solving most serious ecological problems. (For example, most people who get a high LSAT score won’t get into Yale Law. This tells us nothing about whether, for most people, getting a high LSAT score is one of the requirements to get into Yale Law.)