Step 1: Understanding the Concept:
The Yajurveda is known as the 'Veda of rituals'. It provides the prose mantras and instructions required by the Adhvaryu priest, who performs the manual labor and movements of the Vedic sacrifice. A major historical division in this Veda is between the Shukla (White) and Krishna (Black) recensions.
Step 2: Detailed Explanation:
Assertion (A) is correct. The very name 'Yajur' comes from the root 'yaj', meaning to sacrifice. While the Rigveda provides the hymns (Riks) and the Samaveda provides the melodies (Samans), it is the Yajurveda that provides the 'Yajus' (prose formulas) that accompany the physical actions of the Yajna.
Reason (R) is also a correct factual statement regarding the difference between the two branches. The Shukla Yajurveda (Vajasaneyi Samhita) is called "White" or "Pure" because it contains only the Mantras. The ritual explanations (Brahmanas) are kept separate in the Shatapatha Brahmana. In contrast, the Krishna Yajurveda (like the Taittiriya Samhita) is called "Black" or "Mixed" because the Mantras are interspersed with the prose explanations and ritual instructions (Viniyoga-vakyas).
However, (R) does not 'explain' (A). The fact that the Krishna Yajurveda is mixed and the Shukla is pure is a structural detail of the text's organization. It is not the reason why the Yajurveda as a whole is concerned with the sacrifice. The Yajurveda's concern with sacrifice is due to its functional role in the Vedic liturgical system (the Trayi). The internal arrangement of mantras and prose within its branches is an independent literary characteristic.
Step 3: Final Answer:
Both statements are individually true facts of Vedic literature, but the internal structural difference (R) is not the causal reason for the general ritualistic nature (A) of the Veda.