The correct answer is option 3 - Yajurveda.

The Yajurveda is one of the four Hindu scriptures and is an important text in ancient Indian literature. It primarily consists of hymns and rituals to be performed during religious ceremonies.

The earliest literary references to iron in the Indian subcontinent are found in the Yajurveda. These references indicate that iron was known and used during this time. Iron has been an essential material in human civilization, enabling the development of tools and weapons that played a significant role in progress and warfare.

While the Rigveda and Samaveda are also important ancient Indian scriptures, there are no specific mentions of iron in them. Similarly, the Vinaya Pitaka, which is a part of the Buddhist canon, does not contain references to iron.

Hence, based on the available information, the earliest literary references to iron in the Indian subcontinent are indeed found in the Yajurveda.