1 ____ = 1% of viewport height.

The unit of measurement that defines 1% of the viewport height is the "vh" unit. This is part of a set of relative length units in CSS that are based on the size of the viewport (the visible area of the web page). Specifically, 1vh equals 1% of the height of the viewport. For example, if the height of the viewport is 1000 pixels, 1vh would be equal to 10 pixels. This unit is particularly useful for responsive design, as it allows elements to scale in size relative to the current height of the viewport, ensuring consistency across different screen sizes.

In contrast, other options like "vmin" and "vmax" relate to the smaller or larger of the viewport height or width, not specifically defining a percentage of height. "rem" is a length unit that is relative to the root font size and does not connect directly to viewport dimensions. Thus, "vh" is the correct choice, as it clearly signifies a measurement based on the viewport's height.