Answer:
C. balance.
Step-by-step explanation:
Balance is not specifically a form of health-related fitness, also can indicate several aspects, it could be muscular balance, cholesterol balance, electrolyte balance; it just doesn't indicates an aspect of improvement through fitness training.
On the other hand, options A, B and D, indicate a specific health-related fitness, for example, cardiovascular fitness is aimed to improve cardiorespiratory system, muscular endurance to improve muscle's capacity, body composition refers to key percentages and ratios as fat, bone, water and muscle.
That's why option C is the right answer.