The correct answer is option 1: Parsees. The Zend-Avesta is indeed the sacred book of the Parsees, who are followers of Zoroastrianism. The Zend-Avesta is a collection of religious texts and scriptures that contain teachings and rituals of Zoroastrianism, an ancient monotheistic religion that originated in Persia (modern-day Iran). Zoroastrianism was founded by Zarathustra (also known as Zoroaster), who is considered the prophet of this faith. The Zend-Avesta is divided into several sections, including the Yasna (which includes the Gathas, hymns composed by Zarathustra himself) and the Visperad (ritual prayers). It is considered a sacred and authoritative text for Parsees, who continue to uphold the teachings and practices of Zoroastrianism.