Explanation:

The Vedas are a large body of religious texts originating in ancient India, forming the oldest layer of Sanskrit literature and the foundational scriptures of Hinduism. * **Correct Option (A) Rigveda:** The Gayatri Mantra is one of the most sacred and significant mantras in Hinduism. It is found in Mandala 3, Hymn 62, Verse 10 of the **Rigveda**, which is the oldest of the four Vedas. The mantra is dedicated to Savitr, the solar deity, and is a prayer for spiritual enlightenment and wisdom. Its profound spiritual significance makes it a central part of many Hindu rituals and meditations. * **Incorrect Option (B) Yajurveda:** The Yajurveda primarily consists of prose mantras and verses used for rituals and sacrificial ceremonies. While it contains numerous mantras, the Gayatri Mantra's original source is not the Yajurveda. * **Incorrect Option (C) Samveda:** The Samveda is known for its melodies and chants, largely derived from the Rigveda, which are set to music for liturgical purposes. Although it draws heavily from the Rigveda, the original composition of the Gayatri Mantra is in the Rigveda itself. * **Incorrect Option (D) Atharvaveda:** The Atharvaveda contains hymns, spells, and charms, often related to healing, protection, and daily life, and is distinct in its content from the other three Vedas. The Gayatri Mantra is not found in the Atharvaveda.