The correct answer is option 'D', Saraswati river.

Explanation:
The Rig Veda, the oldest text in Hinduism, contains hymns and prayers composed by the Indo-Aryans during the Rig-Vedic age (1500-1200 BCE). The Rig Veda mentions several rivers, and one of the most prominent and revered rivers in the Rig Veda is the Saraswati river.

1. Importance of Rivers in Rig Vedic Age:
- Rivers held great significance in the Rig Vedic age and were considered divine entities.
- They were praised and worshipped, and their waters were believed to possess purifying and life-giving properties.
- Rivers played a crucial role in the daily life and religious rituals of the Rig Vedic people.

2. Naditarna - The Best River:
- Among the rivers mentioned in the Rig Veda, the Saraswati river is referred to as "Naditarna," which means the best river.
- Saraswati is praised as a mighty and sacred river, flowing from the mountains to the ocean.
- It is described as a life-giving river, providing abundance and prosperity to the people.

3. Saraswati River:
- The Saraswati river is believed to have been a major river in the northwestern region of the Indian subcontinent.
- It is often associated with the present-day Ghaggar-Hakra river system, which once flowed through what is now the Thar Desert in Rajasthan.
- The Saraswati river holds immense religious and cultural significance in Hinduism and is considered the goddess of knowledge, music, and arts.

4. Decline and Disappearance:
- The Saraswati river gradually declined and disappeared due to natural geological changes, including tectonic shifts and changes in the course of rivers.
- Over time, the river changed its course, dried up, or became a seasonal river, leaving behind the Ghaggar-Hakra river system.
- The decline and disappearance of the Saraswati river had a profound impact on the Rig Vedic society and influenced the migration and settlement patterns of the Indo-Aryans.

In conclusion, the Saraswati river was referred to as "Naditarna" or the best river in the Rig Veda. It held great importance and was praised as a sacred and life-giving river. Despite its decline and disappearance, the Saraswati river continues to hold cultural and religious significance in Hinduism.