Final answer:
The appropriate form of the verb ser to fill in the blank in the sentence 'La clase de español. ______ muy divertida ayer' is 'es'.
Step-by-step explanation:
The appropriate form of the verb ser to fill in the blank in the sentence 'La clase de español. ______ muy divertida ayer' is es. This is because it is referring to the current state of the Spanish class, which is ongoing and continuing from the past to present. The verb ser is used to express essential characteristics or identity, and in this case, it describes the nature of the Spanish class.
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