Answer:
the production of good Y, and stat trading with Small Nation in order to obtain good Y.
Step-by-step explanation:
Big country's opportunity cost of producing goods X instead of good Y = 60/80 = 0.75 good Y per good X
Big country's opportunity cost of producing goods Y instead of good X = 80/60 = 1.33 good X per good Y
While Small Nation's opportunity cost of producing either good X or good Y = 60/60 = 1
Since Big Country's opportunity cost of producing good Y is higher than Small Nation's opportunity cost, then Big Country should give up the production of good Y, and instead trade with Small Nation to obtain good Y.