You are a cake baker.
Employers are bakery owners. They pride themselves as establishments where people can visit to buy delicious cakes to enjoy. To be the best bakery on the block, they want to hire the best bakers.
It can be a risky business hiring bakers because if they aren’t good at baking cakes, they can slow down the other chefs on the team and end up producing low-quality cakes. As a result, they decided that when vetting applications, they would like to see a few cakes baked by the applicant before hiring them.
That cake is a case study in your design portfolio. Employers are looking for beautiful and delicious cakes. If the cakes you put on display looks nice, and smells good, they will likely believe you have great baking skills.
Most bakeries sell more than one type of cake, so you may want to display more than one type of cake in your “portfolio”.
Generally you want to pick different cakes, as three strawberry shortcakes would only be interesting if you are selling yourself as a specialist in strawberry cakes. Likely you will find the most success if you pick a few cake types that the bakery you want to work for sells.