The following two images show a web page that uses a responsive layout displayed on...

First drop-down: flex boxes

Second drop-down: background-size

Third drop-down: cover

The layout changes from a stacked mobile layout to a multi-column desktop layout without relying on media queries. That behavior is commonly achieved with responsive flex boxes, where items can wrap naturally based on the available width. The header image must fill the available header area on both mobile and desktop. For that requirement, the correct CSS property is background-size, and the correct value is cover. background-size: cover scales the image so the entire header area is filled, even if part of the image is cropped. contain would preserve the full image but could leave empty space, and auto would keep the image at its natural size.