Correct answer:
a. era; era
Era is the conjugation of the verb ser for the third person singular in the imperfect tense. This tense is used to talk about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. The first part of the sentence cuando yo era niño uses the verb ser as condition because it expresses something that varies over several years. On the other hand, the second part of the sentence mi casa era azul uses the verb ser as description because it talks about the essential quality that describes a house such as its color. Finally:
Cuando yo era niño, mi casa era azul.
