Final answer:
Based on the provided chart, Martha swam for the longest time on Thursday, with 84 hours spent swimming.
Step-by-step explanation:
The subject question asks about the length of time Martha swam each day during her vacation. The times listed are in a sequential increasing order: Monday (81 hours), Tuesday (82 hours), Wednesday (83 hours), and Thursday (84 hours). The question seeks to identify the day on which Martha swam the longest.
From the data provided, we simply compare the hours swam on each day. Monday is 81 hours, Tuesday is 82 hours, Wednesday is 83 hours, and Thursday is 84 hours. With each day representing a sequential increase by one hour, it's clear that Thursday is the day Martha swam the longest, with 84 hours.