What CSS property controls the layout of flexbox elements?

The property that controls the layout of flexbox elements is indeed "display: flex;". This property is essential for creating a flexible box layout. When you apply "display: flex;" to a container element, it transforms that container into a flex container, allowing its child elements to be positioned and resized according to the flexbox model.

By using "display: flex;", the children of the container automatically become flex items. This means they can easily be manipulated with various other flexbox properties, such as "flex-direction," "justify-content," and "align-items," which determine the direction of the layout, alignment within the flex container, and distribution of space among the items. However, none of these properties would have any effect if "display: flex;" were not applied to the container in the first place.

In contrast, the other options listed—such as "flex-direction," "flex-layout," and "layout-mode"—are either specific properties that modify the behavior of flex items within a flex container (like "flex-direction" which sets the direction of the flex items), or do not exist in standard CSS terminology. Therefore, understanding that "display: flex;" is the foundational property to utilize flexbox layout is crucial.