What best explains the linguistic diversity in the Caucasus region and New Guinea?

The linguistic diversity in the Caucasus region and New Guinea can best be explained by both areas being mountainous regions, which create isolated environments or "cultural pockets." Mountainous terrain often leads to geographical barriers that hinder communication and interaction between groups. As a result, populations that live in proximity to one another can develop distinct languages and dialects over time.

In the case of the Caucasus, its complex mountain ranges contribute to numerous languages and ethnic groups coexisting in relatively close quarters, fostering unique linguistic developments. Similarly, New Guinea is one of the most linguistically diverse areas in the world, and its rugged geography has facilitated the emergence of over 800 distinct languages, as many communities remained isolated from one another. This concept of isolation is integral to understanding the development of languages in both regions.

In contrast, the other choices do not adequately address the reasons behind the linguistic diversity. For example, while coastal regions might suggest cultural diffusion, this diffusion is less relevant in these mountainous areas where physical barriers limit such interactions. Similarly, stating that they are tropical or extensive plains regions does not hold true for the Caucasus and misrepresents the natural landscapes involved.