The correct answer is option 'C', Vedic Samhitas.

Vedic Samhitas are the oldest texts of the Hindu religion and form the foundation of Hindu philosophy and rituals. They are the oldest layer of the Vedas and contain hymns, prayers, and rituals. The Samhitas are divided into four collections: Rigveda Samhita, Samaveda Samhita, Yajurveda Samhita, and Atharvaveda Samhita.

The royal sacrifices of Rajasuya, Asvamedha, and Vajapeya are first mentioned in the Vedic Samhitas. These sacrifices were elaborate and prestigious rituals performed by kings during ancient times.

- Rajasuya Sacrifice:
The Rajasuya sacrifice is a royal consecration ritual. It was performed to establish the king's sovereignty and authority. The ritual involved the king being anointed as the emperor or the paramount ruler. The Rajasuya sacrifice is mentioned in the Rigveda Samhita (10.16) and the Yajurveda Samhita (TS 4.1.10.1-10).

- Asvamedha Sacrifice:
The Asvamedha sacrifice is a horse sacrifice performed by a king to display his power and dominance. It involved the king releasing a specially chosen horse and allowing it to wander freely for a year. If the horse was unchallenged and returned to the king's kingdom, it symbolized his sovereignty over the surrounding territories. The Asvamedha sacrifice is mentioned in the Yajurveda Samhita (TS 7.1.1.1-5) and the Shatapatha Brahmana.

- Vajapeya Sacrifice:
The Vajapeya sacrifice is a ritualistic drinking of Soma, a sacred drink. It was performed by a king to gain strength and vitality. The ritual involved the king drinking Soma, accompanied by chanting of hymns and offering of oblations. The Vajapeya sacrifice is mentioned in the Rigveda Samhita (8.25-31) and the Yajurveda Samhita (TS 1.1.11.1-8).

These royal sacrifices were significant events in ancient Indian society and marked the power and status of the ruling kings. They were performed with great pomp and were considered to bring prosperity and blessings to the kingdom.

In conclusion, the royal sacrifices of Rajasuya, Asvamedha, and Vajapeya are first mentioned in the Vedic Samhitas, which are the oldest texts of the Hindu religion. These sacrifices played an important role in establishing the authority and power of the kings and were performed with great grandeur and significance.