The Avesta is a collection of sacred texts in Zoroastrianism, an ancient Iranian religion. It is composed of various texts, including hymns known as Gathas. These Gathas are considered to be the oldest and most important part of the Avesta, as they are believed to have been composed by Zoroaster himself. The Avesta also contains teachings about the Amesha Spentas, who are divine beings associated with various aspects of creation, and Ahura Mazda, the supreme deity in Zoroastrianism. Therefore, the Avesta is the correct answer as it is the collection of texts that includes the Gathas, Amesha Spentas, and Ahura Mazda.