The correct answer is option 'A', Rigveda.

Rigveda is one of the oldest sacred texts of Hinduism, and it is divided into ten books called Mandalas. Each Mandala consists of hymns, prayers, and rituals that were composed by different sages or Rishis. Gayathri Mantra is one of the most widely known and revered mantras in the Rigveda.

The Gayathri Mantra is found in Book 3, Hymn 62, Verse 10 of the Rigveda. It is dedicated to the Sun God, Savitr, and is considered a powerful mantra for spiritual enlightenment and divine knowledge. The mantra is written in the Sanskrit language and is composed of 24 syllables.

The Gayathri Mantra is as follows:

ॐ भूर्भुवः स्वः।
तत्सवितुर्वरेण्यं
भर्गो देवस्य धीमहि।
धियो यो नः प्रचोदयात्॥

Om Bhur Bhuvaḥ Swaḥ
Tat Savitur Vareṇyaṃ
Bhargo Devasya Dhīmahi
Dhiyo Yo Nah Prachodayāt

The mantra is a prayer for the divine light of the Sun to illuminate our intellect and guide us on the path of righteousness. It is often chanted during meditation and in various religious ceremonies.

The Gayathri Mantra is considered to be of great significance in Hinduism and is believed to have the power to purify the mind and soul. It is also believed to bestow wisdom, knowledge, and spiritual awakening upon the practitioner.

In addition to its presence in the Rigveda, the Gayathri Mantra is also found in other ancient Hindu scriptures such as the Samaveda, Yajurveda, and Atharvaveda. However, its primary and most well-known appearance is in the Rigveda.

Overall, the Gayathri Mantra holds a special place in Hindu religious practices and is revered for its profound spiritual significance.