Final answer:
The correct answer is 'A. era.'
Step-by-step explanation:
The correct answer is A. era. In the sentence, 'Cuando yo era niño, quería ser bombero,' the verb 'ser' should be conjugated in the imperfect form to match the past tense description of being a child. The imperfect form of the verb 'ser' in the first-person singular (yo) is 'era.' This indicates that when the speaker was a child, they wanted to be a firefighter.
Learn more about Conjugating the verb 'ser' in the imperfect form