The correct answer is: A. Rigveda
The Rigveda is the oldest Veda, and one of the four sacred texts of Hinduism. It is a collection of Vedic Sanskrit hymns and religious texts, believed to have been composed in India between 1500 and 1200 BCE. The Rigveda is the most important source of information about the early history of Hinduism, and it contains many references to the Saraswati river.
The Saraswati river is a sacred river in Hinduism, and it is mentioned in many Hindu texts. The Rigveda contains the earliest references to the Saraswati river, and it describes it as a mighty river that flows through the land. The Rigveda also contains hymns to the Saraswati river, and it praises its beauty and power.
The Saraswati river is no longer a major river, and it is believed to have dried up in the 2nd millennium BCE. However, the Saraswati river is still a sacred river in Hinduism, and it is mentioned in many Hindu texts. The Rigveda is the oldest and most important source of information about the Saraswati river, and it contains many references to the river.
The other options are incorrect because they are not the oldest Veda. The Samaveda, Atharvaveda, and Yajurveda are all later Vedas, and they do not contain as many references to the Saraswati river as the Rigveda.