What does the display property set to flex do for an element?

Setting the display property to flex allows an element to use the flexbox layout model, which is designed for organizing its children in a flexible and efficient manner. This means that the children elements can be laid out in rows or columns, depending on the specified flex-direction property.

Using flexbox enables responsive design, as it allows elements to adjust their size and position based on the available space, making it easier to create fluid layouts. Consequently, items within a flex container can grow, shrink, and reorder dynamically, providing a powerful way to manage layout compared to traditional block or inline styles.

The other options refer to properties or behaviors that do not apply when the display property is set to flex; for instance, hiding an element or adding borders are separate CSS functionalities, and making an element block-level does not relate to the flexible layout capabilities that flex provides.