The correct answer is C. Oxygen.
Oxygen is the most abundant element in the Earth’s atmosphere, making up about 21% of the air we breathe. It is also the third most abundant element in the Earth’s crust, making up about 46% of the mass of the planet. Oxygen is essential for life, as it is used in cellular respiration to produce energy. It is also used in many industrial processes, such as the production of steel and glass.
Calcium is the fifth most abundant element in the Earth’s crust, making up about 3.6% of the mass of the planet. It is a major component of bones and teeth, and it is also used in many industrial processes, such as the production of cement and fertilizer.
Silicon is the second most abundant element in the Earth’s crust, making up about 28% of the mass of the planet. It is a major component of sand and rocks, and it is also used in many industrial processes, such as the production of glass and semiconductors.
Nitrogen is the seventh most abundant element in the Earth’s atmosphere, making up about 78% of the air we breathe. It is also a major component of proteins and nucleic acids, and it is used in many industrial processes, such as the production of fertilizer and explosives.