During aerobic activity, if your heart rate is lower than the lower limit, you are not pushing yourself hard enough pushing yourself to the limit at just the right spot pushing yourself too hard

Not pushing yourself hard enough is the answer since your heart rate doesn't even hit your lower minimum.
Pushing yourself to the limit is at your max heart rate.
Just at the right spot is at your max heart rate.
Pushing yourself too hard is above your max heart rate.

The answer is b hope this