If I had __known she was ill ,I would have visited her

and和but是连接词 排除掉

still和yet

yet多用于否定句
“already”用于肯定句，“yet”用于否定句。
如：
1) He has already finished his homework.
2) I have not finished reading the book yet.
注意：“already”用于疑问句时，表示惊奇。
如：Have you already seen him?

still既可用于肯定句又可用于否定句，表示动作或状态的持续。
如：
1) She still likes him.
2) I still can not understand you.
注意：“still”置于动词前后，会使句意发生变化。
如：
1) He is still standing. (continue to stand)
2) He is standing still. (=standing motionless)

简单点就是
still一般在句子中间
I still do my homework
yet一般在句尾
I haven't done my homework yet

所以选c

C.still

a b 是绝对没可能的,意思根本就不对.

d的话,yet的意思也不对.

这句的意思是,如果我知道她病的话,我会过去看望她的.