Text:

लोकादिमग्निं तमुवाच तस्मै या इष्टका यावतीर्वा यथा वा ।
स चापि तत्प्रत्यवदद्यथोक्तंअथास्य मृत्युः पुनरेवाह तुष्टः ॥ १५ ॥

lokādimagniṃ tamuvāca tasmai yā iṣṭakā yāvatīrvā yathā vā |
sa cāpi tatpratyavadadyathoktaṃathāsya mṛtyuḥ punarevāha tuṣṭaḥ || 15 ||

15. Death told him the fire, the source of the worlds what altars (to be raised), how many and how, and Nachiketas repeated it all as explained. Then Death, being delighted, said to him again.

Shankara’s Commentary:

This, the Sruti itself says. Death explained o Nachiketas, the fire, which was the source of the worlds, being the first embodied existence, the same that was asked for by Nachiketas; again he told him of what form the sacrificial bricks were to be, how many in number and in what manner the sacrificial fire was to be lit and all this; Nachiketas also faithfully repeated what was told him by Death. Then delighted by his repetition, Death said to him again inclined to grant him a boon other than the three promised.