What is the use of em in CSS?

The unit "em" in CSS is a relative unit of measurement that is based on the font size of the element to which it is applied. One em is equivalent to the current font size of that element. For example, if an element has a font size of 16 pixels, then 1 em would be equal to 16 pixels. If you specify padding or margin in ems, such as 2 em, it would be 32 pixels in this case.

Using ems allows for greater flexibility and scalability in responsive design. As the font size changes, the measurements in ems will adjust accordingly, making it easier to create layouts that adapt to different screen sizes or user settings. This relative measurement is particularly useful for maintaining consistency in design across various devices and user preferences.

Other choices identify units or functions in CSS that do not relate to the nature of ems or describe them inaccurately, focusing instead on fixed measurements or unrelated properties.