Final answer:
The correct pronoun to complete the second sentence is 'lui'.
Step-by-step explanation:
The correct pronoun to complete the second sentence is lui. In French, when referring to people, the pronoun 'lui' is used to mean 'to him' or 'to her'. Therefore, Sylvie is saying 'Alors, tu dois lui donner un cadeau!', meaning 'So, you have to give her a gift!'
Learn more about French Pronouns