Final answer:
d) Agriculture and grain constituted the industry representing three-fourths of the middle colonies' exports due to their long growing seasons and fertile lands, with the export of grains like wheat being particularly important for the economy.
Step-by-step explanation:
The industry which accounted for three-fourths of the exports from the middle colonies was agriculture and grain. In the middle colonies, farmers utilized the long growing season and fertile land to grow a variety of grains such as wheat, rye, oats, barley, buckwheat, and corn. This was supported by the fact that grain, especially wheat and flaxseed, became the most important commodity due to the region's ability to sell grain to both internal and external markets.
Furthermore, labor patterns in the middle colonies were largely shaped by family farms worked predominantly by free labor, which allowed the region to meet both agricultural and manufacturing demands. As farming was a major economic activity, the sale of grain products became vital for the middle colonies' export trade.