1. The student who is dressed in white is her daughter.

→The student dressed in white is her daughter.  

2. I have read plenty of books which were written by Lu Xun.

→I have read plenty of books written by Lu Xun.  

3. His nephew,  who was lost at sea when he was fifteen,  had been his only relative.

→His nephew, lost at sea when he was fifteen,  had been his only relative.