b - her
c - their
a- It's my mother's umbrella. It's her umbrella.
b- It's my father's car. It's his car.
c- It's my parent's house. It's their house
d- They are the children's cassettes. They are their cassettes
e- It's the old lady's bag. It's her bag.
a- Whose dress is this ? It's Helen's dress.
b- Whose socks are they ? They are the boy's socks.
c- Whose record is this ? It's my father's record.
d- Whose computer is this ? It's the Simpsons' computer.