Answer:
1) The girl is my cousin who is smiling at us.
2) The gardener is chopped down the tree that had been attacked by white ants
3) The teacher spoke to you just now who is new to our school
4) Did you notice the man who was walking with a limp
5) I know the boy whose father was killed in a car accident
6) The man was sitting in the car just now who was waiting for his daughter
Step-by-step explanation
Here you can find the word that you have to use exactly like this,
Who is usually use for when we reference people/animals not things
Whom is usually use for pronouns like him/her /us ect.
Whose is usually use for possessive pronouns like his / our ect.
Which /That are usually use for when we reference things not people or animals
hope this helps you.
Please let me know if you have another questions :-)