The correct answer is - Trade.
The Inca Empire was a great civilization that was well developed. The empire occupied high mountainous parts, but also lowlands, and coastal parts. The road infrastructure throughout the empire was on a high level and that was for multiple reasons, trade, communication, travel.
The people that lived in the mountains were producing certain products, and the people in the lowlands and coastal areas other products. Because of the great road infrastructure, the free movement of goods throughout the empire was relatively easy, so the potatoes could've easily go to the coastal areas, and the fish up in the mountainous areas.