Final answer:
To complete the sentence, use the pretérito perfecto de indicativo form of the verb dejar.
Step-by-step explanation:
To complete the sentence in the pretérito perfecto de indicativo, we need to conjugate the verb dejar in the third person singular form for Julian. The pretérito perfecto de indicativo of dejar in the third person singular is ha dejado. So the completed sentence would be: Julian ha dejado de tomar refrescos.
Learn more about Spanish verb conjugation