Option a is the correct answer. Bhagavad Gita, the Brahma Sutra, the Mukhya Upanishads are known collectively as the Prasthan trayi. These three books provide a foundation for major influential monistic schools of Hinduism. They are authoritative and primary sources that teach the means to strive for salvation. Hence, Bhagavata is not the part of Prasthan trayi.