Final answer:
To complete the sentence, use the third-person plural conjugation of the verb ser: 'son'
Step-by-step explanation:
To complete the sentence "Yolanda y Diana ", we need to choose the correct conjugation for the verb ser. In this case, since the subject is 'muchachas' (which means 'girls'), we need to use the third-person plural form of ser. Therefore, the conjugation we should use is 'son'. The complete sentence would be: 'Yolanda y Diana son muchachas felices' (Yolanda and Diana are happy girls).
Learn more about Conjugation of the verb ser