Answer:
C. 2.67 bushels of corn
Explanation:
The opportunity cost is the cost of the best  alternative not taken. In this case, if you choose to produce beans, you choose not to produce corn (this is the bets altenative not taken.. in this case the only one alternative).
For every 6 bushels of beans, you choose not to produce 16 bushels of corn, so:
6 bushels of beans= 16 bushels of corn
1 bushel of beans = (16 bushels of corn/6)
1 bushel of beans= 2.67 bushels of corn
