Right answer:
b. rubio
Since we are describing the skin color of Julieta, it is obvious that we need to describe the skin color of Nicolás as well. So Julieta is brunette while Nicolás is blond. This sentence is given in the imperfect tense. This tense is used 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. Here the verb estar stands for the third person singular in the imperfect, whose conjugation is era. Finally:
Julieta era morena y Nicolás era rubio
