Complete the sentences with the correct relative pronoun or adverb

1. We are talking about the boy
   [which|when|who] won the competition.
2. We saw the house
   [whose|when|where] ​Agatha Christie lived.
3. She doesn't know
   [whose|who|what] letter it was.
4. The ice-cream
   [what|which|whose] I brought is in the fridge.
5. He knows the reason
   [which|that|why] Helen is so upset.