Six kg is a measure of an object's:

The measure of six kilograms pertains to the object's mass. Mass is a fundamental property of matter that quantifies the amount of substance within an object. It remains constant regardless of the object’s location in the universe. Mass is typically measured in kilograms or grams, making the choice of six kg an indication of mass.

Weight, on the other hand, refers to the force exerted by gravity on an object and is dependent on both mass and gravitational acceleration. Therefore, if discussing weight, the measurement would be expressed in newtons rather than kilograms.

Volume refers to the amount of space an object occupies and is measured in liters, cubic meters, or similar units. Length is a measure of distance and is typically measured in meters or centimeters. Since six kg directly quantifies how much matter is present in an object, it is accurately described as its mass.