1.  That's the teacher ______ helped me most.

2.  The museum ______ we visited was fascinating.

3.  He is someone ______ opinion I really value.

4.  My brother (non-defining: extra info — he is a doctor) lives in Porto.

5.  The student ______ got the highest mark won a prize. (defining)

6.  Paris, ______ is the capital of France, is beautiful.

7.  The girl which helped me was very kind.

8.  London, that is the capital of England, is very expensive.

9.  That's the place where I bought it. (referring to a book)

1.  who

2.  where

3.  whose

4.  My brother, who is a doctor, lives in Porto.

5.  who

6.  which

7.  The girl who helped me was very kind.

8.  London, which is the capital of England, is very expensive.

9.  That's the book that I bought.