In restrictive (defining) relative clauses, omission of the relative pronoun is syntactically possible under specific conditions. The sentence in which the relative pronoun could be grammatically omitted without altering the structure of the sentence is:
- A this is the book which I was talking about
- B Mr Jones, who is sitting in that corner, is the managing director
- C that is the hotel in which I stayed last holiday
- D what was the name of the person to whom you spoke on the phone