The correct answer is A. Argon.
Noble gases are elements in group 18 of the periodic table. They are very stable and do not react with other elements. Argon is the most abundant noble gas in the Earth’s atmosphere, making up about 0.93% of the air. It is also used in many industrial applications, such as lighting and welding.
Nitrogen is the most abundant element in the Earth’s atmosphere, making up about 78% of the air. It is also a component of many fertilizers and explosives.
Oxygen is the third most abundant element in the Earth’s atmosphere, making up about 21% of the air. It is essential for life, as it is used in respiration.
Fluorine is a highly reactive element that is not found in the Earth’s atmosphere. It is used in many industrial applications, such as refrigerants and plastics.