Correct Answer:
C. Oxygen
The correct answer is Oxygen because it comprises nearly half of the entire chemical mass composition of the Earth's crustal layer.
- Oxygen: This is the correct choice. Oxygen makes up approximately 46.6% of the Earth's crust by weight. It is not found as a free gas here, but rather chemically bound within silicate minerals, oxides, and water.
- Incorrect Options:
- Carbon dioxide / Ozone: These are incorrect. These choices represent atmospheric gases rather than elements that constitute the solid, physical crust layer of the Earth. Silicon is the second most abundant crustal element, not carbon or ozone variants.