Final answer:
Jeff's heart rate would be 90 bpm to indicate he was hardly working, as it is lower than his THR zone of 135-185 bpm.
Step-by-step explanation:
Jeff's Target Heart Rate (THR) zone is 135-185 beats per minute (bpm). If Jeff's heart rate is to indicate that he was hardly working, it would likely be lower than the target zone. Therefore, the answer would be (a) 90 bpm, as this value is significantly lower than the minimum of the THR zone, suggesting minimal exertion.
To indicate that Jeff was hardly working, his heart rate would be below his target heart rate (THR) zone of 135-185 bpm. Among the given options, the closest heart rate that falls below his THR zone is 130 bpm, so the correct answer would be option d) 130 bpm.