Step 1: Understanding the Concept:
The question asks to identify the source of a specific quote that uses an analogy of words and their union to describe the process of education involving a teacher, a pupil, and knowledge. This metaphorical style is characteristic of the Upanishads.
Step 2: Detailed Explanation:
The quote is a direct reference to a passage in the Taittirīya Upaniṣad. Specifically, it is from the third Anuvāka (section) of the first chapter, known as the Śikṣā Vallī (Chapter on Instruction).
In this section, the Upanishad presents a series of "great connections" (mahāsaṃhitāḥ) through analogies. The passage mentioned in the question is one such analogy, where:
The teacher (ācārya) is the first form (pūrvarūpam).
The pupil (antevāsī) is the latter form (uttararūpam).
Knowledge (vidyā) is their junction (sandhiḥ).
Instruction (pravacanam) is the connection (sandhānam).
This illustrates the sacred and interconnected nature of the teaching-learning process. The other texts listed do not contain this specific formulation.
Step 3: Final Answer:
The quote is explicitly from the Śikṣā Vallī of the Taittirīya Upaniṣad. Therefore, option (2) is correct.