Concept:
Extinction means the complete dying out of a species so that no individuals remain anywhere. This directly reduces the variety of living things on Earth.
Step 1:
Biodiversity is the variety of all living species. When a species goes extinct, the total number of species falls, which is exactly a loss or depletion of biodiversity.
Step 2:
Air pollution, climate change and waste production are environmental problems that may contribute to species loss, but the term extinction itself is most directly an expression of biodiversity depletion.
Answer: Option (4) — Biodiversity depletion.