On his way to school he met ------,so he sent him to hospita

首先,这个问题本身就有语法错误.每一个选项前都应该有一个a.冠词.
ill是表语形容词,不能放名词前面,可以说the man's very ill,而不能倒过来放前面.相反,sick就可以.所以说a very sick man正确.
much作副词的时候有更的意思,只能修饰形容词比较级.
serious是形容词,不能修饰形容词.