Which of the following is NOT a primary component of physical fitness?

The choice indicating respiration rate as NOT a primary component of physical fitness is accurate because respiration rate itself is not typically categorized as a core component of physical fitness. Instead, it is a physiological measure that can be influenced by various factors, including fitness levels and health conditions.

The primary components of physical fitness usually include cardiovascular endurance, muscular strength, and flexibility. Cardiovascular endurance refers to the ability of the heart and lungs to supply oxygen to the muscles during prolonged physical activity. Muscular strength relates to the amount of force a muscle can produce, which is essential for performing various physical tasks. Flexibility involves the range of motion in joints and can impact overall physical capability and injury prevention.

Respiration rate is an important aspect of overall health and can provide insights into a person's fitness level, but it does not stand alone as a component of physical fitness compared to the others listed. It is more of a vital sign that can reflect cardiovascular and respiratory system efficiency. Thus, it is clear why it is classified separately from the primary components of physical fitness.