Question

Which is more fundamental, the mass of a body or its weight ? Why ?

Answer 1

Mass is fundamental. It is a dimensionless quantity which is a measure of amount of a substance.
Weight is actually force, i.e. weight = mass x acceleration (due to gravity).
So, weight is different according to difference in acceleration due to gravity.

Mass is more fundamental because it is constant for an object. The only way an object can have more or less mass is if you add more or less mass to it. The weight of an object can change based on the force of gravity. This is why an object will weigh less on the moon than on the Earth.