Question

as a result of the development of interchangeable parts, the southern region’s economy relied on...
cash crops
ship building
food crops
livestock

Explanation:

The Southern economy before industrialization (and with interchangeable parts' impact contextually, though the key is historical economic reliance) was centered on cash crops (like cotton) for trade/profit. Ship building is more Northern/industrial, food crops were for sustenance, livestock less dominant. So the Southern economy relied on cash crops.

Answer:

Cash crops