Explanation:
For this particular species I would label 90°F to 94°F as the temperature zone of physiological stress.
We find that within this temperature range the number of individual in the population lessens. This means they are under certain physiological stress which limit there growth.
Again within 95°F–96°F we find that the number of individual is maximum. This means that the population can utilize the available resources to their maximum efficiency at this range. So, it is the optimum range.