Jawaban:

1 Alexandra Cousteau, who is a conservationist, believes water will be the most important issue of the 21st century.

2 Alexandra Cousteau, whose grandfather was an oceanographer, is a great advocate for environmental protection.

3 Cousteau says that the environmental movement uses communication tools which are old-fashioned.

4 She is currently planning an expedition which aim is to research different water ecosystems around the world.

5 She is currently working on a book which she hopes will teach people how to live on a 'Water Planet'.

6 The problem that she says was created by people is a problem which people can only solve by working together.  

Penjelasan:

RELATIVE PRONOUNS

1) WHO

Digunakan untuk menjabarkan orang (people) dan berfungsi sebagai pengganti posisi subyek.  

Contoh:

The girl who spoke about the boy is standing over there.

The boy, who was angry at his friend, bit his hand.

A 70-year-old man, whose house was recently struck by lightning, said he lost everything.

2) WHICH / THAT

Digunakan untuk menjabarkan kata benda dan hewan. Berfungsi pengganti posisi subyek, obyek.

Contoh:

Give me some food which  I may eat.

3) THAT

That digunakan sebagai kata ganti atau relative pronoun dan dapat dipadukan dengan jenis noun seperti manusia, hewan dan benda mati. That juga memiliki fungsi yang netral yang bisa menggantikan “who/which/whom”.

4) WHOSE

Digunakan untuk memodifikasi kepemilikan obyek. Whose menggantikan possessive adjective determiner [her, his, their and etc]

Contoh:

That is the little girl whose mother was killed in traffic accident.

Semoga membantu ya.