Answer:
1) The girl is my cousin <u>whose</u><u> </u>smiling at us.
2) The gardener is chopped down the tree <u>that</u><u> </u>had been attacked by white ants
3) The teacher spoke to you just now <u>who</u><u> </u>is new to our school
4) Did you notice the man <u>who</u><u> </u> was walking with a limp
5) I know the boy <u>whom</u><u> </u>father was killed in a car accident
6) The man was sitting in the car just now <u>who</u><u> </u>was waiting for his daughter