She reached a conclusion that there was no more she could do.

reached 是动词 reach 的过去式，reach 本意是"到达、达到"，引申为"得出（结论）"。 a 是不定冠词，表示"一个"。 conclusion 是名词，来自动词 conclude（结束、得出结论），意思是"结论"。 reached a conclusion 意思是"得出了一个结论"。