Answer:
c. Decisions are made at the margin.
Explanation:
It is better to make a trade-off when someone compares the costs with that of the benefits of doing something. The [tex]\text{decisions about whether one must do a bit more or a bit less of an activity}[/tex] are called marginal decisions. Making the [tex]\text{trade-offs at the margin}[/tex] means comparing its costs as well as its benefits of an activity by doing it a little bit more versus doing a little bit less.
In the context, Ana is making decision at the margin when she decides to practice one hour extra and improve her swimming performance as the event of triathlon is approaching.
Thus, option (d) is correct.