Choose the sentence that has the correct form of the verb ser. (4 points)
question 5 options:
1) ellas es muy inteligentes.
2) clara no es cubana.
3) nosotros son amigos.
4) yo eres serio.

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- son

So now you just have to identfy the sentence where both the noun/pronoun matches the verb conjugation.

Therefore the best answer would be Clara no es cubana (option 2).  

It could not be:
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