Answer:
a. 9 pounds of fish per pound of cheese.
Explanation:
The gain from trade would arise at the time when the price of the cheese with respect to the fish is more than 5 and less than 10
So according to the given option the first option is met the criteria
The 9 is more than 5 and less than 10
And, the other options does not met the criteria
So, the first option i.e. a is correct
And, all other options are wrong