What physical characteristic does the moment of inertia of a rotating object most directly and accurately measure?

radius

mass

momentum

rotational inertia

It is within the definition of moment of inertia itself that rotational inertia appears. Just like some people interpret an object's mass as a resistance to changes in its linear motion, some people interpret an object's moment of inertia (around a given axis) to be its resistance to changes in its rotational motion around that axis.