Hello!
¿Sabes dónde están mías llaves? is incorrect.
The correct form uses mis: ¿Sabes dónde están mis llaves?
Mía is the equivalent of "mine" while mis translates to "my". That is why mis is correct here - you wouldn't ask "Where are mine keys?".
Este libro es de Carolina. Es suya.
Suyo and suya are possessive pronouns; pronouns take the place of people.
Su and sus are possessive adjectives; adjectives describe nouns.
Caroline is a person, therefore the answer must be either suyo or suya. Since she is female, suya is more correct.
