This biodiversity boosts the oxygen availability

The correct option is Ecosystem diversity.

To understand why this is the correct answer, let us break down the concepts of biodiversity and look at how different levels of diversity impact the environment, particularly oxygen availability.

Biodiversity is typically categorized into three main levels:
1. Genetic diversity: The variety of genes within a single species.
2. Species diversity: The variety and abundance of different species within a region.
3. Ecosystem diversity: The variety of different ecosystems, habitats, and ecological processes in a biosphere or a large geographic area.

An ecosystem consists of all the living organisms interacting with their physical, non-living environment (such as water, soil, and air). When we have high ecosystem diversity, it means a region contains a variety of distinct ecosystems, such as tropical rainforests, wetlands, oceans, grasslands, and deciduous forests.

Each of these distinct ecosystems contains unique communities of primary producers (like trees, grass, phytoplankton, and algae) that perform photosynthesis. Photosynthesis is the chemical process by which green plants and other autotrophs use sunlight to synthesize nutrients from carbon dioxide and water, releasing oxygen as a byproduct.

The chemical equation representing photosynthesis is:
$$6{\mathrm{CO}}_2+6H_{2}O\to C_6{H}_{12}{O}_6+6O_2$$

Different ecosystems contribute to global and local oxygen cycles in complementary ways. For instance, terrestrial forests generate significant atmospheric oxygen, while marine and freshwater ecosystems (driven by phytoplankton and aquatic plants) produce a massive portion of the Earth's oxygen. Because different ecosystems function together to drive biogeochemical cycles on a large scale, it is the variety and stability of these diverse ecosystems (Ecosystem diversity) that ultimately boosts and maintains global oxygen availability.