What does setting the display property to 'flex' accomplish in CSS?

Setting the display property to 'flex' in CSS enables a powerful two-dimensional layout system. This allows a parent element to manipulate the layout of its children in both rows and columns, making it easier to create complex layouts with less code compared to traditional methods such as floats or positioning.

When an element is set to 'display: flex', its immediate children (the flex items) can be aligned and distributed along the main axis (horizontal) or the cross axis (vertical) with a variety of alignment and justification properties. This flexibility means that elements can shrink, grow, or be spaced within the flex container in a dynamic way, responding automatically to the size and space available, leading to responsive designs.

While the other choices touch on aspects of layout or positioning, they do not accurately capture the broad functionality provided by flexbox. For instance, vertical alignment can be achieved with flexbox, but that is a specific feature rather than the primary purpose. Similarly, while ordering of elements can be adjusted with flex properties, this is just one feature among many that flexbox offers. Lastly, setting display to 'flex' doesn’t convert elements to block-level; it creates a flex container that may affect the layout of its children. Thus, the correct understanding of '