Theoretically, if a person or a country had an unlimited amount of a resource, then that person or country would:
a. have to place limits on its use.
c. run out of it.
b. not have to make a choice on how to use it.
d. have to worry about supply

An unlimited amount means that there would always be more of it: so they can't run out of it, there will always be more! and since they can't run out of it, they don't have to worry or make choices - they can just use it without any limits and thinking about it.

So the correct answer is
a. have to place limits on its use.