The correct completion of the sentence would be:
Cuando era niño, yo hablaba mucho con mis hermanos.
How to fill the blanks?
The verb "hablar" is conjugated in the imperfect tense for the first person singular (yo). In Spanish, for regular -ar verbs like "hablar," the imperfect tense is formed by removing the -ar ending and adding the appropriate imperfect tense endings. For "yo," the ending is "-aba."
Therefore, "yo hablaba" is the correct imperfect form, and the sentence translates to "When I was a child, I used to talk a lot with my siblings." The imperfect tense is often used to describe habitual or repeated actions in the past.