Step 1: Understand what megadiverse means.
A megadiverse country is one that holds an unusually large share of the world's plant and animal species within its borders.
Step 2: Check India against that definition.
India spans deserts, rainforests, mountains, coasts and wetlands, and this variety of habitats supports species found nowhere else on earth.
Step 3: Rule out the other options.
Population size, industrial output and climate uniformity have nothing to do with biological richness, so they cannot explain the megadiverse status.
\[ \boxed{\text{Wide range of ecosystems and endemism}} \]