An _______ is the size of type as computed relative to the type size of the parent element.

The term that fits in the blank is "em." This unit of measurement is relative to the font size of the parent element in CSS. If a parent element has a font size of 16 pixels, one "em" would be equal to 16 pixels. This makes "em" a versatile unit for responsive design, as it allows text and other elements to scale proportionally based on the parent element's font size. Thus, when using "em," if you change the parent’s font size, all child elements using "em" units will also scale accordingly, providing a flexible and fluid typographic layout.

In contrast, the other options serve different functions in typography and styles: "pt" (points) is a fixed unit commonly used in print media; "rem" (root em) is relative to the root element's font size rather than the parent element; and "pc" (picas) is another fixed unit used primarily in print design, not influenced by surrounding elements' sizes. Understanding these differences is crucial for effective web design and ensuring consistent visuals across various devices and screen sizes.