Final answer:
The three main principles of fitness - overload, specificity, and progression - apply to improving flexibility, making Option C) Flexibility the correct answer.
Step-by-step explanation:
The three main principles of fitness - overload, specificity, and progression - are key components to improving various physical abilities. When it comes to improving your flexibility, these principles still apply. Applying the concept of overload means you must stretch your muscles beyond their usual length to improve flexibility. Specificity relates to focusing on the type of flexibility required for specific movements or activities. Progression refers to gradually increasing the difficulty or intensity of the flexibility exercises over time to continue making improvements.
Therefore, the three main principles of fitness directly apply to improving the flexibility described in Option C) Flexibility.