The correct answer is not option 'B' but option 'C' - Upanishads.

Early mystics in India drew upon a variety of scriptures in their discussions and teachings. However, the Upanishads are considered to be the primary source of mystical knowledge and insight.

The Upanishads are a collection of ancient Sanskrit texts that contain some of the central philosophical and spiritual ideas of Hinduism. They are considered to be the culmination of the Vedas, the oldest sacred texts of Hinduism.

The Upanishads are characterized by their focus on the nature of reality, the self, and the ultimate goal of human existence. They explore the relationship between the individual self (atman) and the universal self (Brahman), and offer insights into the nature of consciousness, the nature of God, and the nature of the universe itself.

Early mystics, such as the sages who founded the Upanishadic tradition, drew upon the insights and teachings of the Upanishads to develop their own mystical practices and insights. They used the Upanishads as a framework for exploring the nature of reality and the self, and for developing techniques for achieving spiritual enlightenment and liberation.

In addition to the Upanishads, early mystics also drew upon a variety of other scriptures, including the Vedas, the Brahmanas, and the Dharmasutras. However, the Upanishads remain the most important and influential source of mystical knowledge and insight in the Hindu tradition.