The polar regions, also known as the Arctic and Antarctic, have a sparse population primarily due to unsuitable climatic conditions. Option 2 correctly identifies this as the reason for the low population in these areas. The extreme cold temperatures and harsh weather conditions make it challenging for human habitation. The Arctic region experiences long, frigid winters with limited daylight, while the Antarctic is subjected to continuous darkness for months. The lack of sunlight and extreme weather make it difficult to grow crops or raise livestock, resulting in limited resources for sustenance. Additionally, the polar regions have a limited infrastructure and lack amenities necessary for a large population. While options 1, 3, and 4 may contribute to the challenging conditions in the polar regions, the main factor causing their sparsely populated nature is option 2.