Step 1: Concept Identification:
The query seeks the origin of a quote that uses a word-union analogy to depict education involving a teacher, student, and knowledge. This allegorical style is characteristic of the Upanishads.
Step 2: Detailed Analysis:
The quote directly references a passage within the Taittirīya Upaniṣad, specifically from the third Anuvāka of the first chapter, known as the Śikṣā Vallī (Chapter on Instruction).
This section details "great connections" (mahāsaṃhitāḥ) through analogies. The quote is one such analogy, where:
The teacher (ācārya) is the initial form (pūrvarūpam).
The student (antevāsī) is the subsequent form (uttararūpam).
Knowledge (vidyā) serves as their union (sandhiḥ).
Instruction (pravacanam) constitutes the connection (sandhānam).
This formulation underscores the profound and interconnected nature of the teaching-learning process. The alternative texts provided do not contain this specific phrasing.
Step 3: Conclusion:
The quote originates from the Śikṣā Vallī of the Taittirīya Upaniṣad. Consequently, option (2) is correct.