The correct answer is: A. Low reactivity with metal.
Argon is a noble gas, which means that it is very stable and does not react easily with other elements. This makes it ideal for use in arc welding, where it can protect the metal from oxidation and other contaminants.
Option B is incorrect because argon does not lower the melting point of metal. In fact, it has the opposite effect and can actually raise the melting point of some metals.
Option C is incorrect because argon is not flammable. It is a non-flammable gas that is used in many applications where fire or explosion hazards are present.
Option D is incorrect because argon does not have a high calorific value. Calorific value is the amount of heat produced by the combustion of a unit mass of a substance. Argon has a very low calorific value, which means that it does not produce much heat when it is burned.