6. Mary is here. Where are ... ?

The sentence is asking for the location of Mary. The pronoun 'are' is used with the plural subject 'you' or when referring to multiple people or things. Since the question is about Mary, and the context implies a singular subject, the missing word is likely 'you' if it's a direct question to someone about Mary's whereabouts, or it could be a typo and intended to be 'is' if asking about Mary's location directly. However, given the options are not provided, and assuming a standard question structure, 'you' is the most common completion for 'Where are ... ?' when asking someone else.