Final answer:
The correct verb form to fill in the blank is 'murió' (option B).
Step-by-step explanation:
The correct answer is option B.
Juan está muy triste porque su perro murió la semana pasada.
The verb form 'murió' is the appropriate past tense form of 'morir' (to die) in Spanish, which matches the context of the sentence.
Learn more about Verb forms in Spanish