Tropics have greater biodiversity because:

The correct answer is Option (2) → More solar energy is available in the tropics

The latitudinal gradient is the best-known pattern of biodiversity: species diversity decreases as we move away from the equator towards the poles. Ecologists give three main reasons why the tropics are richer: a long, undisturbed history, a less seasonal and more predictable climate, and more solar energy.

Option (1) Tropical environments are less predictable — Incorrect. The fact is reversed. Tropical environments are less seasonal, more constant and more predictable than temperate ones. Such constant conditions favour niche specialisation and so more species.

Option (2) More solar energy is available in the tropics — Correct. More sunlight gives higher productivity in the tropics. This in turn may add, indirectly, to greater diversity.

Option (3) Tropical regions have been more subjected to glaciation in the past — Incorrect. This too is reversed. It is the temperate regions that faced frequent glaciations. The tropics have remained relatively undisturbed for millions of years, which gave them a long time for new species to form.

Option (4) There is low productivity in the tropics — Incorrect. The tropics have high productivity, because they receive more solar energy. Low productivity would support fewer species, not more.