Final answer:
The correct verb to fill in the blank is 'estaba.' It refers to the temporary anger of Juan's mother when he returned home.
Step-by-step explanation:
The correct verb to fill in the blank is estaba.
The sentence is saying that Juan's mother was very angry when he returned home. This suggests that her anger was temporary and a specific event triggered it, which is why the past progressive verb 'estaba' is the appropriate choice.
For example: La madre de Juan estaba muy furiosa cuando él regresó a casa.
Learn more about Verb usage in Spanish