Answer:
Option (A) is correct.
Step-by-step explanation:
Here, the principle states that there are many economics decisions that can be made by analyzing the cost and benefit associated with that decision.
In these kind of decisions, a person looking at the cost and benefit associated with an additional or extra unit consumed or produced.
In our case, her husband wants to spend an extra hour on the swimming pool sessions as this would improving his his swimming skills and hence the chances of winning the triathlon also there.