Final answer:
The correct verb form to fill in the blank is 'dije'.
Step-by-step explanation:
The correct verb form to fill in the blank in the sentence 'Yo ______ que el concierto fue muy bueno.' is dije, which corresponds to option B. This is the correct form of the verb 'decir' (to say) in the past tense for the first person singular (yo).
Learn more about Verb conjugation in Spanish