I think it is c.He felt ill, but he had a fever
b. because the conjunction 'but' is used when stating something that contrasts with the first part of the sentence. The second part of sentence b. supports the first part. The conjunction used in that sentence should be 'and'.
All sentences use coordinating conjunctions correctly, but option B may be seen as less effective due to a mismatch in implied sentiment. Therefore, option B is the best choice regarding ineffective use. The sentence suggests a contrast that doesn't align perfectly with its ideas.
;