Which factor most likely caused animals and plants in india to differ greatly from species in nearby southeast asia?

The two biggest reasons for the differences in the flora and fauna between India and Southeast Asia, despite being so close to each other are:

- Indian was a separate continent;

India was a separate landmass for millions of years, thus the evolution of the species on it went in isolation, creating new unique species, but also preserving some of the prehistoric ones. Even after the collision with the Eurasian plate, lot of those species managed to survive and prosper.

- There's a big natural barrier between the regions;

Between India and Southeast Asia there's very high mountain ranges. These mountain ranges have acted as a barrier for the movement of the species from one place to another, as not lot of species are able to cross high mountains in order to migrate to new territory.

the correct answer is - ability to find a mate