Great question! Believe it or not, S, Dawn Embers, and Elfycia Lee ☮ are all correct. The bottom line is, it depends on your goal.

If you're just looking to win the Prep and walk away with a badge and a prepped novel, a scene is fine. It's not easy writing a complete story in fifteen minutes, but fifteen minutes is all that's required. If you spend fifteen minutes but didn't actually get a complete story out, it counts. You get credit, and you're done if you're happy.

If you want to truly understand your protagonist better, only a complete story will show you how your character reacts to conflict, and how they have grown and evolved in the past when faced with conflict.

If you want to WIN the contest round, then it depends completely on (1) the preference of the judges, and (2) the competition. If everybody writes a scene without a discernible plot, your chances of winning with a scene are obviously higher. But it's likely that others who also want to WIN the contest round recognize that a complete story is more engaging and satisfying to any reader, including your judges, and will provide them with a complete story accordingly. The judges' instructions are simply to pick the entries the like best, and the only criteria for entry qualification are (1) the entry must be submitted by noon WDC time on Sunday, and (2) the story must be accessible by the judges (privacy set to Authors+ or Group Only).

Weirdly, I don't think anyone has asked this question in the seventeen years we've been running the Prep. But the fact is, I created the contest rounds specifically to be this flexible. I wanted to entice more competitive writers to participate in PrepMo, but without scaring away less competitive writers who just need a little motivation and organization.

Cheers,
Michelle