Answer: Option (a) is correct.
Step-by-step explanation:
Correct Option: Lower than his opportunity cost of that good.
Opportunity cost is the benefit that is foregone for an individual by choosing one alternative over other alternatives available to him.
If the opportunity cost is lower for an individual then this will benefit him whereas if the opportunity cost is higher then this will not benefit the individuals.
In our case, if people obtained a good at a price that is lower than his opportunity cost of that good then he will be benefited from the trade.