Final answer:
The correct sentence using the relative pronoun correctly and with proper punctuation is: 'Everybody was talking about the robbery, which happened last week.'
Step-by-step explanation:
The correct sentence using the relative pronoun correctly and with proper punctuation is:
Everybody was talking about the robbery, which happened last week. The relative pronoun 'which' is used correctly here to provide additional information about the robbery. A comma is used before the relative pronoun to separate it from the main clause. This sentence structure indicates that the information after the comma is not essential to the meaning of the main clause.
Learn more about Relative pronouns in English grammar