Why might the region of South Asia be referred to as "a land of great variety"?
South Asia has a great deal of variety in a number of ways.
For example, South Asia is home to a variety of physical environments. You have, for example, jungles in India, the Himalaya mountains, and the deserts of Afghanistan, all of which are in South Asia.
As another example, you have a variety of religions in this area. You have Hinduism and Jainism and Sikhism, all centered in India. You have Islam in Indian and Pakistan and Afghanistan as well as other places. You have Buddhism, which started in India and is still practiced in some areas of Nepal, for example.
There is also a great deal of ethnic diversity in this area with all sorts of ethnicities in India, in Pakistan and in Afghanistan.
In all of these ways, South Asia is a land of great variety.