Which of the following measurements is relative to the size of its parent element?

The measurement that is relative to the size of its parent element is "em." This unit is particularly useful in responsive web design because it allows elements to adapt to changes in font sizes dynamically. When you set an element's font size using ems, it will scale based on the font size of its parent container. For example, if a parent element has a font size of 16px and you set a child element's font size to 1.5em, the child will inherit this size as 24px (1.5 times 16px).

This scaling approach facilitates better flexibility and maintains proportion among text elements, particularly in fluid layouts. It's a handy tool for ensuring that text remains readable and appropriately sized across varying screen resolutions and device types.