Final answer:
The frequency of cardiorespiratory training primarily depends on an individual's fitness level. This determines how often they should engage in such activities to maintain or improve their health and performance.
Step-by-step explanation:
The frequency of cardiorespiratory training depends on several factors, but among the options provided: age, fitness level, nutrition, and clothing, the correct answer is b) Fitness level. Individuals with different levels of fitness will require different frequencies of cardiorespiratory exercise to achieve or maintain an optimal level of health and performance. While other factors like age and nutrition can also influence how often a person should engage in cardiorespiratory training, fitness level is a direct indicator of the body's ability to handle and recover from exercise stress. Clothing is more related to comfort and safety during the exercise rather than the frequency of the workouts.