Correct option is B
Krishna preached the Bhagavad Gita to Arjuna in the Bhishma Parva of the Mahabharata.
Additional Information:
The Bhagavad Gita appears in Chapters 23 to 40 of the Bhishma Parva.
It is the conversation between Lord Krishna and Arjuna on the battlefield of Kurukshetra, just before the war begins.
In this discourse, Krishna explains the concepts of duty (dharma), karma, yoga, and the nature of the self (atman).