Step 1: Understanding the Concept:
This question deals with the definition of 'Veda' from the perspective of Sayana, the great commentator on the Vedas. In the introduction to his commentary on the Taittiriya Samhita, Sayana provides two distinct ways to define the Veda: one based on its origin (Ontological) and another based on its function (Teleological).
Step 2: Detailed Explanation:
Assertion (A) refers to the ontological definition: "Apauruṣeyaṃ vākyaṃ vedaḥ" (The Veda is a sentence/word that is not of human origin). In Indian philosophy (especially Mimamsa and Vedanta), the Vedas are considered eternal (Nitya) or revealed, but not composed by any human intellect. This establishes their absolute authority and freedom from human errors.
Reason (R) refers to the functional definition: "iṣṭaprāptyaniṣṭaparihārayoralaukika-mupāyaṃ yo grantho vedayati sa vedaḥ" (The book which makes known the supernatural means of obtaining the desired and avoiding the undesired is the Veda). This defines the Veda by its purpose—to provide knowledge that cannot be gained through sensory perception or logic (Alaukika Upaya).
While both statements are correct definitions provided by Sayana, (R) is not a logical 'explanation' of (A). The fact that a text shows supernatural means (Function) is not the reason why it is not of human origin (Ontology). One could theoretically have a humanly composed book that claims to show supernatural means (though it wouldn't have the same authority). The "Apaurusheya" nature is about the source of the text, whereas the "Ishta-prapti" definition is about the content/utility of the text. They are two parallel definitions of the same entity.
Step 3: Final Answer:
Both definitions are traditionally valid and correct, but they address different aspects of the Veda. Therefore, (R) does not explain (A).