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