Introduction:
The question asks for the source that does not mention anything about trade routes of ancient India. In order to answer this question, we need to examine each of the given sources and determine which one does not discuss trade routes.

Milinda Panho:
The Milinda Panha is a Buddhist text that consists of a dialogue between the Indo-Greek king Menander I and the Buddhist sage Nagasena. This text does mention the trade routes of ancient India. In one section, Nagasena describes the various routes traveled by merchants and the goods that were traded. Therefore, option 'a' is not the correct answer.

Sangam literature:
Sangam literature refers to a collection of Tamil literature composed between the 1st and 3rd centuries CE. While this literature primarily focuses on topics such as love, war, and politics, it also provides valuable insights into the trade and commerce of ancient India. Many poems in Sangam literature mention the trade routes and the commodities that were exchanged. Therefore, option 'b' is the correct answer as it does not discuss trade routes.

Jataka stories:
Jataka stories are a collection of tales that recount the previous lives of the Buddha. These stories often include references to trade and commerce. They provide insights into the economic activities of ancient India and mention the trade routes that connected different regions. Therefore, option 'c' is not the correct answer.

Periplus of the Erythrean Sea:
The Periplus of the Erythrean Sea is an ancient Greek text that provides a detailed account of trade routes and harbors in the Indian Ocean. It describes the maritime trade between Egypt, India, and other regions. This source extensively discusses the trade routes of ancient India, making option 'd' an incorrect answer.

Conclusion:
After analyzing each of the given sources, we can conclude that option 'b' (Sangam literature) does not speak about trade routes of ancient India.