The correct answer is option 2: Mass is a constant of proportionality.
In physics, weight and mass are two different concepts. Mass is the measure of the amount of matter in an object and is a fundamental property of the object. It does not change with the force of gravity applied to the object. Mass is measured in kilograms.
Weight, on the other hand, is the force exerted on an object due to gravity. It depends on the mass of the object and the acceleration due to gravity. Weight is measured in newtons. The weight of an object can vary depending on the strength of the gravitational field it is in. For example, an object will weigh less on the moon compared to the Earth due to the difference in gravitational pull.
Therefore, option 1 is incorrect because weight is not a constant of proportionality as it can change depending on the gravitational field. Option 3 is also incorrect as mass is indeed a constant property of an object. Option 4 is incorrect as weight is not a universal constant, it can vary depending on the location.