1. Me llamo Carolina. Yo soy una pasajera. - you have to use the article una because it refers to female gender. Un is used for males, unas is used for females, but in plural.
2. Ana y Susana, son ustedes unas turistas? - here, you have to use unas because Ana and Susana are females, and there are two of them, meaning you need the plural form of una, which is unas
3. Manuel es un conductor de autobus. - here you have to use un because Manual is a man; unas and una refer to females
4. Senora Castillo, hay un problema con las maletas? - even though problema looks as if it were female gender (usually nouns which end in -a are female), in Spanish that noun is male, which is why you have to use un
