1/3 bushel of rye
Explanation:
Given that the farmer produces 30 bushels of corn every year but using the entire field on producing 30 corn bushels would cost 10 rye bushels. Thus, the opportunity cost of producing one bushel of corn would be 1/3 bushel of rye(10/30). The slope of the farmer's PPF would be -1/3 and hence, the opportunity cost of the farmer can be calculated by considering the reciprocal of a bushel of corn's opportunity cost.