Final answer:
The best interrogative word to use in the sentence is 'Por qué' to inquire about the reason or cause behind the action.
Explanation:
The best interrogative (question word) to complete the question is 'Por qué.' In this sentence, the question is seeking the reason or motive behind a particular action – going to buy something promptly. Using 'Por qué' helps to inquire about the cause or reason for the action.
For example: '¿Por qué quieres ir a comprar tan pronto?' translates to 'Why do you want to go buy so soon?'
Remember that 'Por qué' is used to ask questions related to reasons or causes.
Learn more about Interrogative Words in Spanish here:
https://brainly.com/question/48759573
