Ответ:

Перевод задания: Вставьте which, where, who, whose или when.

Defining relative clauses:

  1. I like parties which have a theme.
  2. Jane is the person who I think throws the best parties.
  3. The festival which I like the most is Christmas.

Non-defining relative clauses:

  1. Katie, whose brother is an actor, is getting married next week.
  2. March 8th, when International Women’s Day takes place, is my birthday.
  3. Buñol, where La Tomatina is held every year, is a small town in Valencia.
  4. Mehmet, who is Muslim, celebrates Ramadan.

В defining clauses местоимения who/which в предложениях 1–3 можно опустить, так как они являются дополнениями.