The correct answer is: C. em
An em is a relative unit of measurement in typography that is equal to the height of the current font’s capital M. This means that an em is always relative to the font size, so if you change the font size, the em size will also change. This makes ems a convenient unit for measuring things like the size of headings and paragraphs, as they will always be proportional to the font size.
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 is a unit of measurement that is absolute, and an ex is a unit of measurement that is equal to half the height of the current font’s capital M.