<p>(Not many) < A> authors (have described)<b> the effects of environmental pollution (as effective as)<c> Rachel, whose work is still (a model for)<d> nature writers.</d></c></b></p><b>
<p>Why C?</p>
</b>
<p>It should read:
Not many authors have described the effects of environmental pollution as effectively as Rachel, whose work is still a model for nature writers.</p>
<p>“Effectively” is an adverb that modifies the verb “describe,” whereas “effective” is an adjective.</p>