The Nagar style of architecture developed mostly in North India. This style of architecture is commonly found in the regions of Punjab, Haryana, Himachal Pradesh, and Uttar Pradesh. It is characterized by its tall and slender temples with intricate carvings and richly adorned facades.

Option 1: South India - This is not the correct answer. The Nagar style of architecture is not prevalent in South India. South Indian architecture has its own distinct style known as Dravidian architecture.

Option 3: North-West India - This is not the correct answer. While North-West India has its own architectural styles, the Nagar style is not one of them.

Option 4: North-East India - This is not the correct answer. The Nagar style of architecture is not found in North-East India. This region has its own unique architectural styles influenced by the tribal cultures of the area.

In conclusion, the Nagar style of architecture developed mostly in North India, particularly in states like Punjab, Haryana, Himachal Pradesh, and Uttar Pradesh.