He became aware that he had lost his audience since he had not been able to talk coherently.

【正确答案】 C

【答案解析】 副词词义辨析。根据句意，lost his audience的原因是说话不流利了。选项中，intelligibly“可理解地，清楚地”，与划线词语意思相近，其他选项与句意不符。故答案为C。