Caroline is training for a triathlon, a timed race that combines swimming, biking, and running.

Consider the following sentence: Each hour she spends swimming is an hour that she can't spend biking or running.

Which basic principle of individual choice does this sentence best illustrate?

Each hour she spends swimming is an hour that she can't spend biking or running. The basic principle this sentence illustrates is:

All choices have opportunity cost.

Explanation:

As per the given situation, Caroline decides to go for swimming. So the time she invests in swimming, she could have done for biking or running during that time. So, opportunity cost of one hour of swimming is an hour of biking or running. Also it is not known that whether Caroline has an incentive if she spends more time swimming. This applies for an hour spent for biking or running as well. Thus, all the three choices have an opportunity cost.