Final answer:
The correct pronoun in this sentence is 'their'.
Step-by-step explanation:
The correct pronoun to use in this sentence is their.
When there are two or more people mentioned as the subject, we use the pronoun 'their'. In this case, Lauren and Christina are the subjects and they are both taking their lunch out to the park.
For example: Lauren and Christina took their lunch out to the park.
Learn more about Pronouns