The correct answer is: D. ex
An ex is a unit of length in typography that is equal to the height of the lowercase “x” in a given font. It is used as a relative unit of measurement, meaning that it is always based on the font size. For example, if a font has an x-height of 10pt, then 1ex would also be 10pt.
The other options are not relative units of measurement. A percentage (%) is a unit of measurement that is relative to the size of the parent element. A centimeter (cm) is a unit of measurement in the metric system. A point (pt) is a unit of measurement in typography that is equal to 1/72 of an inch.
Here is a table that summarizes the different units of measurement and their definitions:
| Unit of measurement | Definition |
| — | — |
| % | A percentage is a unit of measurement that is relative to the size of the parent element. For example, if a parent element has a width of 100px, then a child element with a width of 50% would have a width of 50px. |
| cm | A centimeter is a unit of measurement in the metric system. It is equal to 1/100 of a meter. |
| pt | A point is a unit of measurement in typography. It is equal to 1/72 of an inch. |
| em | An em is a unit of measurement in typography that is equal to the width of the uppercase “M” in a given font. It is used as a relative unit of measurement, meaning that it is always based on the font size. |
| ex | An ex is a unit of measurement in typography that is equal to the height of the lowercase “x” in a given font. It is used as a relative unit of measurement, meaning that it is always based on the font size. |