The correct answer is "quis", meaning "who" or "what." The sentence translates to "Who is Flavia?" It should be remembered that the verb "to be" takes the nominative and not the accusative, thus eliminating all the other options, except for "quid" which is neuter and not used for people, and "quae" which is plural.

The noun oppidum is neuter and in this case is in the nominative singular. The correct demonstrative pronoun that agrees with a neuter nominative singular noun is hoc.