The correct answer is D. vh.
A viewport is the area of a web page that is visible in a browser window. The height of the viewport is the height of the browser window minus the height of the browser’s toolbars and status bar.
A vh unit is a relative unit of measurement that is equal to 1% of the height of the viewport. This means that a vh unit will always be the same size, regardless of the size of the browser window.
The other options are not relative units of measurement. A ch unit is a relative unit of measurement that is equal to the height of the current font. A rem unit is a relative unit of measurement that is equal to the font size of the root element. A % unit is a relative unit of measurement that is equal to a percentage of the parent element’s size.