Final answer:
The correct form of the verb ser to fill in the blank is somos.
Step-by-step explanation:
The correct form of the verb ser to fill in the blank in the sentence 'Yolanda y yo muy buenas amigas' is somos. This is because Yolanda y yo is the subject pronoun for 'we', which requires the conjugated form of 'ser' to be somos. The verb 'ser' is used to indicate identity or characteristics of a noun, and in this case, it is showing that Yolanda and I are good friends.
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