The correct answer is A. rem.
A rem (root em) is a relative unit of measurement in CSS that is relative to the font-size of the root element. This means that if the font-size of the root element is 16px, then a rem unit will be equal to 16px. This makes rem units a very useful way to set font sizes, as they will scale automatically with the font-size of the root element.
The other options are not relative units of measurement. A kd is a unit of measurement used in typography to measure the width of a capital “K”. An rr is a unit of measurement used in typography to measure the height of a lowercase “r”. An hx is a unit of measurement used in typography to measure the width of an “x”.