Which unit is best used for responsive web design?

Using "em" as a unit is considered best for responsive web design because it is a relative measurement that adapts based on the font size of the element or its parent container. This adaptability allows for more fluid and scalable designs that can accommodate various screen sizes and resolutions. When the user adjusts their browser settings or the base font size changes (for accessibility purposes, for example), designs measured in "em" will automatically scale appropriately, maintaining usability and readability.

In contrast, pixels are fixed units that do not scale with user settings, making designs less flexible. Inches and centimeters are physical units that are not practical for web design, as they do not account for the variability in screen sizes and resolutions across different devices. Therefore, "em" proves to be the most effective choice for creating layouts that are responsive and accessible on a wide range of devices.