Final answer:
The largest oxygen reservoir on Earth is the hydrosphere, encompassing all water-containing areas of the planet, with the oceans being the primary component.
Step-by-step explanation:
The largest oxygen reservoir on Earth is the hydrosphere, which includes all the areas that contain water on, under, and over the surface of the Earth. This includes the oceans, lakes, streams, groundwater, and glaciers and ice caps. While the biosphere encompasses a variety of ecosystems and lifeforms, and the atmosphere contains oxygen as a gas, it is the hydrosphere (particularly the oceans) that holds the majority of the Earth's oxygen, mostly in the form of water molecules (H2O). The oceans alone cover about 71% of the Earth's surface and contain 97% of the Earth's water, making them a vast reservoir for oxygen. Therefore, the correct answer to the question is D. hydrosphere.