Answer: Country Y, Alpha
Explanation:
Opportunity cost is the cost of the next best alternative which is foregone. So, for country x the opportunity cost of producing 150 Alpha's is 400 Beta's foregone. While, for country Y the opportunity cost of producing 150 Alpha's is 300 Beta's foregone.
Since, Country Y has to give up less to get the same quantity of Alpha's it has a lower opportunity cost of Alpha.
Thus, it would be best for country Y to produce Alpha as it has a lower opportunity cost in Alpha compared to country x.