If the starting tense is future and in the ellos form and its a regular verb the answer would be comprarían.
what is the audio ? it says based on the audio :)
Answer: The difference between the tenses is when you use them. Pluperfect is commonly used in conversation. Preterit perfect is commonly used in formal writing and literature. In most cases, you're going to use the pluperfect.
Explanation: hope this helped :)
