Which fluid measurement type returns a percentage of the viewport width?

The correct choice is the measurement type that utilizes the viewport width as a basis for its value. This measurement, denoted as "vw," represents a percentage of the viewport width. Specifically, 1 vw equals 1% of the width of the viewport, which means that if the viewport is 1000 pixels wide, 1 vw would equal 10 pixels. This is particularly useful in responsive web design, allowing elements to scale relative to the size of the browser window, providing flexibility and adaptability for different screen sizes.

The other measurement types serve different purposes. For instance, the percentage is often used for various properties but can apply to different contexts, not specifically the viewport's width. The "rem" unit is relative to the font size of the root element, rather than the viewport. Lastly, "vmin" calculates based on the smaller value of either the viewport's height or width, ensuring that elements remain proportional, but it does not directly correspond to the viewport width alone. Thus, "vw" is the specific measurement type that accurately returns a percentage of the viewport width.