Right answer:
c. extrovertido.
In this exercise, we are contrasting two people. These two people are Julieta and Nicolás. Here we are using the imperfect tense that 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. So the verb we are using here is ser and this sentence stands for characteristics, that are personality descriptions of a person. Therefore Julieta was shy while Nicolás was outgoing.