The answer is este it is used as a masculine adjective where as esto is a demonstrative pronoun for masculine and feminine nouns
To make the answering process easy let's first conjugate ser in its possible forms:
Yo- soy
Tu- eres
ella/ello/usted- es
nostros- somos
ellos/ellas/ustedes-<span> son
</span>So now you just have to identfy the sentence where both the noun/pronoun matches the verb conjugation.
Therefore the best answer would be <span>Clara no es cubana (option 2).
It could not be:
</span>1) "Ellas es muy inteligentes" because Ellas matches son
3) "Nosotros son amigos" because Nosotros matches somos
4) "Yo eres serio" because Yo matches soy<span>
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