The correct answer is - D. The arid climates cannot support farming.
The South Asia region is a region where the people, as well as the economy of the countries, are still mostly based and dependent on the agriculture. Because of this, the people have been settling in places that are suitable for farming, like the river valleys, and the regions that are under the influence of the monsoons.
The arid regions, on the other hand, are not densely populated, instead they are very sparsely populated. The main reason is that this places have high temperatures throughout most of the year. The precipitation is very rare and in very small amounts. The soil is of bad quality. All of that makes the arid places not suitable for farming, thus the people avoid them because they can not do the activity from which they depend for food and finances.