B. Matter
The relative mass of an object refers to the amount of matter it contains, which is directly related to its mass. Mass is a scalar quantity and does not depend on the gravitational force acting on the object. In contrast, weight is a force that depends on both the mass of an object and the acceleration due to gravity. Body weight and weight are affected by gravity, and relative matter is not a standard term used in physics. Therefore, "Matter" is the best answer as it represents the relative mass of an object.