Answer:
C). But they can control these pests.
Step-by-step explanation:
As per the question, the most appropriate revision of the underlined phrase would be 'but they can control these pests' as it involves the pronoun 'they' forthrightly refers to 'farmers' and thus, it does not contain any ambiguity and convey a clear and coherent meaning. The other options either employ an incorrect pronoun referent(in options A and B) or either offers an inappropriate meaning(option D) or completely transforms the structure and meaning unnecessarily(option E due to use of 'except'). Thus, option C is the correct answer.