Tropics have greater biodiversity because:

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

Ecologists and evolutionary biologists have proposed various hypotheses; some important ones are:

• Speciation is generally a function of time, unlike temperate regions subjected to frequent glaciations in the past, tropical latitudes have remained relatively undisturbed for millions of years and thus, had a long evolutionary time for species diversification,
• Tropical environments, unlike temperate ones, are less seasonal, relatively more constant and predictable. Such constant environments promote niche specialisation and lead to a greater species diversity.
• There is more solar energy available in the tropics, which contributes to higher productivity; this in turn might contribute indirectly to greater diversity.