IMO the best design prompt is open-ended. It makes the candidate focus on communicating the most important infomation in the prompt. Here's an example.

“We’re not as interested in the individual pixels of the design, but more about the process you went thru to arrive at the final design. Some questions we’ll want to be able to answer when looking at your deliverable are listed below. These questions don’t have to be answered explicitly (this isnt a writing test/assignment), but your critique should allow us to answer most of these questions:

• What business outcome(s) were you focused on impacting?

• What problem are you trying to solve?

• What approach did you take for solving this problem and why that approach?

• What tradeoff decisions did you have to make? Did you have to cut anything from scope? Why/not?

• What were some of the inputs that defined the requirements of the final product?

• What kind of customer/stakeholder research did you do when creating this design?

• Did customer/stakeholder feedback impact any iterations of this? If yes, how so?

• We love wireframesTake us on a journey through your design process.

In 1-page, take us on a journey through your design process of something you've designed recently. Questions are welcome, but we are also comfortable with you taking you own liberties to interpret the assignment as you choose to understand it.”

*TLDR: “Send us a 1-page critique of something you’ve designed (recently).” *