Final answer:
The Indus Valley Civilization is indeed considered the most ancient layer of Hinduism according to Prothero, marking it as the True proto-Hindu civilization that later developed into modern Hinduism.
Step-by-step explanation:
According to Prothero, the assertion that the most ancient layer of Hinduism is the proto-Hindu lost civilization of the Indus Valley is True. The Indus Valley Civilization, centered on cities like Harappa and Mohenjo-Daro, is considered the cradle of Hinduism. Developed around 4500 years ago, it is known as one of the world's oldest religions. Following the decline of Indus cities around 1700 BCE, the Vedic period began with the Aryans' arrival, and the Vedic religion eventually evolved into what we know today as Hinduism. The Vedas, which are the oldest texts of Hinduism, hold a vital place in its philosophy and were composed between 1500 and 900 BCE.