Explanation:

The Gayatri Mantra is one of the most revered and powerful mantras in Hinduism, found in the Rigveda. \nA. Vishwamitra: This is the correct answer. According to ancient Hindu traditions and Vedic texts, particularly the Rigveda (Mandala 3, Hymn 62.10), the Gayatri Mantra is attributed to the revered sage (Rishi) Vishwamitra. He is one of the most prominent and legendary rishis in Hindu mythology, known for his spiritual power and austerities. \nB. Vashishtha: Sage Vashishtha was another highly respected Vedic sage, often depicted as a contemporary or even a rival of Vishwamitra in ancient narratives. While he composed many hymns and is a significant figure in Vedic literature, the Gayatri Mantra is not attributed to him. \nC. Indra: Indra is a prominent Vedic deity, the king of Svarga (heaven) and the Devas (gods). He is a central figure in many Vedic hymns, often praised for his prowess and leadership, but he is a divine entity, not a human composer of mantras. \nD. Parikshit: Parikshit was a legendary Kuru king, the son of Abhimanyu and grandson of Arjuna, from the epic Mahabharata. He lived much later than the Vedic period when the Gayatri Mantra was composed and was a ruler, not a composer of Vedic hymns.