Step 1: Understanding the Concept:
The Aitareya Brahmana provides the ritual commentary on Rigvedic hymns. In its opening section, it explains why certain hymns are chosen for the Dikshaniya Ishti (initiation sacrifice). This specific mantra, "tvam-agne sa-prathā asi", is taken from the Rigveda (5.13.4). The Brahmana uses this verse to establish the primacy and the "encompassing" nature of Agni at the start of the great Agnishtoma sacrifice.
Step 2: Detailed Explanation:
The very first chapter (Prathama Adhyaya) of the First Panchika (Aitareya 1.1) is dedicated to the initiation rites.
1. The Theological Opening: The chapter begins with the declaration that Agni is the first of the gods. As the "mouth" of the gods, Agni is the point of entry for any sacrifice. To sanctify the beginning, an offering is made to Agni and Vishnu.
2. The Function of the Mantra: During the offering of the cake (Purodasha) to Agni, the priest recites "tvam-agne sa-prathā asi". The Brahmana explains that Agni is "wide-extended" (sa-pratha). This means that Agni has the capacity to expand and reach all the other deities of the universe. By invoking Agni with this specific attribute, the sacrificer ensures that his offering will expand from the small ritual hearth to the entire cosmic domain.
3. Location in the Text: This discussion occurs in the very first ritual unit of the Brahmana, which is the First Chapter. The first Panchika follows the sequence of the sacrifice: Chapter 1 deals with the opening libations, Chapter 3 deals with the symbolic rebirth (Diksha), and Chapter 5 deals with the Prayaniya. Thus, any mantra establishing the "primacy" or "opening glory" of Agni is logically located in the Prathame'dhyāye. This specific identification of mantras with their Brahmana chapters is a standard feature of NTA subject-specific papers for Vedic literature. Mastering the "Opening Chapter" of the Aitareya is the first step toward a deeper understanding of Rigvedic ritualism.
Step 3: Final Answer:
The verse "tvam-agne sa-prathā asi" is located in the First Chapter (Prathame'dhyaye) of the first Panchika. Therefore, option (1) is correct.