Which stage of the design thinking process involves creating scaled-down versions of the product or specific features to investigate potential solutions?

The stage of the design thinking process that involves creating scaled-down versions of the product or specific features to investigate potential solutions is called the "Prototyping" stage.

Here are the steps:

1. Empathize: Understand the needs of the user.
2. Define: Clearly articulate the user's needs.
3. Ideate: Generate a range of crazy, creative ideas.
4. Prototype: This is the stage you're asking about. Here, solutions are created, often in scaled-down versions to test and explore the problem solutions.
5. Test: Try out the prototypes with the user and get feedback.

So, the answer to your question is the "Prototyping" stage.