Analysis of Blank (i) and (ii): Lackmuller's Claim
The first part of the sentence details Lackmuller's contention that special interest groups (those not tied to ecological concerns, i.e., corporate or business groups) have affected policymaking in a negative way.
- Blank (i): How have the non-ecological special interest groups affected the process? They must have taken control or gained great power.
- A. marginalized in (Made insignificant) - Incorrect (This is what happened to the other group.)
- C. influential in (Having great sway or power over) - Correct. They are powerful in the process.
- Blank (ii): What happened to the genuine environmental groups (those who actually aim to ensure environmental health)? They must have been suppressed or pushed aside.
- D. vindicated (Cleared of blame) - Incorrect
- E. squelched (To put an end to or suppress) - Correct. If the corporate groups are influential, the environmental groups are suppressed.
- F. lionized (Treated as a celebrity) - Incorrect

Lackmuller's claim is: Corporate groups are influential in the process, and environmental groups are squelched.

Analysis of Blank (iii): The Author's Refutation
The second part of the sentence introduces the author's voice using "however," refuting Lackmuller's claim.

The refutation is based on the fact that Lackmuller "fails to account for the signal achievements environmental groups have effected over the last 20 years-often to the chagrin of big business."

If environmental groups have had signal achievements that anger big business, then Lackmuller's claim that they have been squelched and that the corporate groups always win must be flawed.

The word in Blank (iii) must describe Lackmuller's contention as flawed, mistaken, or narrow in scope.
- G. somewhat tentative (Experimental; not confident) - (Incorrect, the issue is its conclusion, not its certainty.)
- H. rarely myopic (Rarely short-sighted) - (Incorrect, if he's missing major achievements, his view is myopic.)
- I. highly misleading (Giving a false idea or impression) - Correct. Because he ignores all the successes, his conclusion that "We Can't Win" is highly misleading.

Final Solution
The completed sentence is:
According to Lackmuller's latest screed, published under the title, Why We Can't Win at Their Game, special interest groups not nominally tied to ecological concerns have become so influential in (i) the process of environmental policymaking that those groups who actually aim to ensure that corporate profit does not trump environmental health have been effectively squelched (ii). Lackmuller's contention, however, is highly misleading (iii) in that it fails to account for the signal achievements environmental groups have effected over the last 20 yearsoften to the chagrin of big business.

Blank (i): C. influential in
Blank (ii): E. squelched
Blank (iii): I. highly misleading