The Matsya Purana is considered one of the oldest of the eighteen Mahāpurāṇas, a genre of ancient Indian scriptures. It is believed to be the first Purana to have been written, and it contains a wide range of stories, including the tale of the great flood. According to tradition, the Matsya Purana was revealed by the fish incarnation of Lord Vishnu.
Hence, the correct answer is Matsya Purana.