Final answer:
A client's exercise at a level 15 on the Borg Rating of Perceived Exertion scale would likely be perceived as very hard, indicating a high-intensity workout.
Step-by-step explanation:
A client exercising at a level 15 on the Borg Rating of Perceived Exertion (RPE) scale is likely to describe the exercise as very hard. The Borg RPE scale is used to measure the intensity of exercise, where ratings can range typically from 6 (no exertion at all) to 20 (maximal exertion). At level 15, the individual is performing an exercise that is quite demanding physically and approaching high intensity, often associated with aerobic exercise of moderate to high intensity that challenges the cardiovascular system and utilizes a significant amount of oxygen.