Questions: collapse of Indus valley cities Indo-Aryans migrate across India and settle the upper Ganges caste system takes hold in India classic Hinduism takes shape

To place the developments in chronological order, we need to consider the historical timeline of ancient India. Here is the correct sequence:

1. Collapse of Indus Valley Cities: The Indus Valley Civilization, one of the world's earliest urban cultures, began to decline around 1900 BCE. This decline is attributed to various factors, including climate change, tectonic activity, and possibly invasions or migrations.

2. Indo-Aryans Migrate Across India and Settle the Upper Ganges: The Indo-Aryans are believed to have migrated into the Indian subcontinent around 1500 BCE. They settled in the region of the upper Ganges and brought with them the early Vedic culture, which laid the foundation for later developments in Indian society.

3. Caste System Takes Hold in India: The caste system, or varna system, began to take shape during the later Vedic period, around 1000 BCE to 500 BCE. This social stratification became more pronounced as the Indo-Aryans settled and established their dominance in the region.

4. Classic Hinduism Takes Shape: Classic Hinduism, characterized by the development of major texts like the Upanishads and the establishment of key religious concepts and practices, began to take shape around 500 BCE. This period saw the synthesis of Vedic traditions with local beliefs, leading to the formation of what is now recognized as Hinduism.

This chronological order reflects the major developments in ancient Indian history and the evolution of its cultural and social structures.