There were six million visitors in all.That was the ------.

C total 总数,在这里是名词
whole 是形容词,作定语,后面要加名词
all 可以是形容词,副词,代词和名词
其名词意思是 所有一切;全部财产
但应这样用：
John Brown gave his all in the struggle for freedom.
约翰·布朗为争取自由而献出自己的一切
显然,这里应用一个名词
所以选C