The question pertains to the trading preferences of Indian merchants after the third century AD. Each provided option represents a direction or region where Indian merchants could have possibly focused their trade efforts.

Option 1 suggests African trade, but Africa was not the significant interface for trade for Indian merchants during that period.

Option 2 implies West-European trade. But this is unlikely because the fall of the Roman Empire, which comprised major parts of Europe, must have made trading with West-Europe challenging.

Option 3, the correct answer, indicates South-East Asian trade. After the third century AD, the Indian merchants indeed started focusing more on the South-East Asian region, fostering strong economic, cultural, and political linkages.

Option 4 presumes Middle-Eastern trade. While the Middle East was a crucial node for international trade, the question implies an increase in dependence, and the Indian merchants` focus shifted more towards South-East Asia during this era.