Question: Pippalada was an ancient Indian Vedic sage who was known to have written which ancient Sanskrit text, embedded inside Atharva Veda?

Context paragraphs:
Pippalada: Pippalada was an ancient Indian Vedic sage and philosopher in Hindu mythology.  He is known to have written the Prashna Upanishad, which is among the ten mukhya(main) upanishads.  He was an avatar of lord Shiva, and son of sage Dadhichi, who donated his bones to Devtas(Demigods) to provide them a material for making weapons and defeat the Asurs.  He was the founder of Pippalada School of thought, which taught the Atharvaveda.

Prashna Upanishad: The Prashna Upanishad (Sanskrit: प्रश्न उपनिषद् , Praśna Upaniṣad ) is an ancient Sanskrit text, embedded inside Atharva Veda, ascribed to "Pippalada" sakha of Vedic scholars.  It is a Mukhya (primary) Upanishad, and is listed as number 4 in the Muktika canon of 108 Upanishads of Hinduism.

Maitrayaniya Upanishad: The Maitrayaniya Upanishad (Sanskrit: मैत्रायणीय उपनिषद् , Maitrāyaṇīya Upaniṣad ) is an ancient Sanskrit text that is embedded inside the Yajurveda.  It is also known as the Maitri Upanishad (Sanskrit: मैत्री उपनिषद् , Maitrī Upaniṣad ), and is listed as number 24 in the Muktika canon of 108 Upanishads.

Naradaparivrajaka Upanishad: The Naradaparivrajaka Upanishad (Sanskrit: नारदपरिव्राजक उपनिषत् , IAST: Nāradaparivrājaka Upaniṣad) is a medieval era Sanskrit text and one of the minor Upanishads of Hinduism.  The text is attached to the Atharva Veda, and is one of the 20 Sannyasa (renunciation) Upanishads.

Maitreyi: Maitreyi ("friendly one" ) was a Hindu philosopher who lived during the later Vedic period in ancient India.  She is mentioned in the "Brihadaranyaka Upanishad" as one of two wives of the Vedic sage Yajnavalkya; he is estimated to have lived around the 8th century BCE.  In the Hindu epic "Mahabharata" and the Gṛhyasūtras, however, Maitreyi is described as an Advaita philosopher who never married.  In ancient Sanskrit literature, she is known as a "brahmavadini" (an expounder of the "Veda").

Mundaka Upanishad: The Mundaka Upanishad (Sanskrit: मुण्डक उपनिषद् , Muṇḍaka Upaniṣad ) is an ancient Sanskrit Vedic text, embedded inside Atharva Veda.  It is a Mukhya (primary) Upanishad, and is listed as number 5 in the Muktika canon of 108 Upanishads of Hinduism.  It is among the most widely translated Upanishads.

Atharvaveda: The Atharva Veda (Sanskrit: अथर्ववेद, "Atharvaveda " from "atharvāṇas " and veda meaning "knowledge") is the "knowledge storehouse of "atharvāṇas", the procedures for everyday life".  The text is the fourth Veda, but has been a late addition to the Vedic scriptures of Hinduism.

Pranagnihotra Upanishad: The Pranagnihotra Upanishad (Sanskrit: प्राणाग्निहोत्र उपनिषत् , IAST:Pranagnihotra Upaniṣad) is a minor Upanishad of Hinduism.  In the anthology of 108 Upanishads of the Muktika canon, narrated by Rama to Hanuman, it is listed at number 94.  The Sanskrit text is one of the 22 Samanya Upanishads, part of the Vedanta school of Hindu philosophy literature and is attached to the Atharva Veda.  The Upanishad comprises 23 verses.

Stambha: Stambha (also spelled as Skambha) - is used to denote pillar or column.  In the context of Jain & Hindu mythology, it is believed to be a cosmic column which functions as a bond, which joins the heaven (Svarga) and the earth (Prithvi).  A number of Hindu scriptures, including the "Atharva Veda", have references to Stambha.  In the "Atharva Veda", a celestial stambha has been described as an infinite scaffold, which supports the cosmos and material creation.

Sita Upanishad: The Sita Upanishad (Sanskrit: सीता उपनिषत् ) is a medieval era Sanskrit text and a minor Upanishad of Hinduism.  It is attached to the Atharva Veda, and is one of the nine Shakta Upanishads.  It is categorized as a late Upanishad, in which goddess Sita is extolled as the Ultimate Reality of the Universe (Brahman), the ground of Being (Spirituality), and material cause behind all manifestation.  The Upanishad identifies Sita with primordial Prakriti (nature) and her three powers, asserts the text, are manifested in daily life as will (ichha), action (kriyā) and knowledge (jnana).

Supporting paragraphs: Pippalada Prashna Upanishad
Answer: Prashna Upanishad