The correct answer is option 4. Option 4 states that as a result of silk route trade, precious metals like gold and silver flowed from Asia to Europe. This statement is not true. In fact, one of the significant aspects of the silk routes was the exchange of goods, ideas, and cultural practices. While luxury items like silk did flow from Asia to Europe, the trade along the silk routes was not dominated by the flow of precious metals.

Option 1 states that historians have identified several silk routes over land and by sea. This is true. The silk routes were not a single, fixed route but rather a network of interconnected routes that linked different parts of the world.

Option 2 states that silk routes have linked Asia with Europe and Northern Africa. This is also true. The silk routes facilitated trade and cultural exchanges between these regions.

Option 3 states that silk routes existed before the Christian era and thrived almost up to the fifteenth century. This is true. The silk routes were active from ancient times and continued to flourish until the fifteenth century when European exploration and the discovery of new sea routes to Asia led to a decline in the importance of the land-based silk routes.

In conclusion, options 1, 2, and 3 are all true