Final answer:
The earliest civilization to flourish according to the timeline is the Indus Valley Civilization (Harappa and Mohenjo-Daro), which dates back to around 3300 BCE and preceded the Mauryan Empire and the Aryans.
Step-by-step explanation:
The civilization that was the earliest to flourish according to the timeline is the Indus Valley Civilization, represented by B. Harappa and C. Mohenjo-Daro in the options provided. This civilization, also known as the Harappan Civilization, dates back to around 3300 BCE during the Bronze Age and is considered one of the world's earliest urban civilizations. It existed well before the Mauryan Empire, which emerged around 326 BCE, and before the Aryans, who migrated into the Indian subcontinent around 2000 BCE.
Harappa and Mohenjo-Daro were two of the major cities of the Indus Valley Civilization and reached their apex around 2600 BCE. Advanced in urban planning, this civilization was known for its well-organized cities with advanced drainage systems, orderly streets, and standard brick sizes, indicative of their use of weights and measures. The decline of the civilization around 1700 BCE eventually led to the migration of the Aryans into the region.