Final answer:
The correct pronoun to use in the sentence is 'they'. Using gender-neutral pronouns, like they/them/their, is now widely accepted to be inclusive of all gender identities.
Step-by-step explanation:
The correct pronoun to use in the sentence is they. More recently, writers have been replacing gendered pronouns with gender-neutral pronouns, such as they/them/their, when the person's gender is unknown or unimportant. Using they as a singular pronoun is now widely accepted and inclusive of those who fall outside the gender binary.