Which CSS unit is relative to the font-size of the element?

The unit that is relative to the font-size of the element is the "em" unit. This unit is particularly useful in responsive design because it allows for more flexible layouts. When you use "em," its value is determined by the font-size of the parent element, making it scalable depending on the context in which it is used. For example, if a parent element has a font-size of 16px, then 1em would equal 16px within that context. If the size of the text changes or if an element is nested within others with different font sizes, the size of the current element will adjust accordingly, maintaining a proportional relationship to its inherited font size.

In contrast, "px" is an absolute unit that doesn't change based on other elements, "cm" is a physical measurement unit mainly used in print media, and "pt" is also a physical unit traditionally used in typography. These units do not scale responsively based on font size, which is essential for creating flexible web designs that adapt to different screen sizes and user settings.