Flexibility of the hands refers to the range of motion and the ability to move the hands and fingers with ease and precision. It is an important aspect of hand dexterity, which plays a crucial role in performing various tasks that require fine motor skills.

Hand dexterity is essential in many activities of daily living, such as writing, typing, playing musical instruments, and doing intricate tasks like sewing or cooking. It also plays a significant role in professional fields like surgery, dentistry, and painting.

Below are the key points explaining why option 'D' - flexibility of the hands - is the correct answer:

1. Range of motion: Hand dexterity involves the ability to move the hands and fingers in various directions and positions. This includes flexion, extension, abduction, adduction, and rotation of the fingers and wrists. The greater the flexibility of the hands, the wider the range of motion available, allowing for better control and precision in performing tasks.

2. Fine motor skills: Fine motor skills involve the coordinated movement of small muscles in the hands and fingers. These skills are necessary for activities that require precision and control, such as writing, drawing, and playing musical instruments. Hand dexterity, particularly the flexibility of the hands, is crucial for developing and refining these skills.

3. Hand-eye coordination: Hand dexterity is closely linked to hand-eye coordination, which is the ability to synchronize visual input with hand movements. This coordination is essential for tasks that require precise hand movements, such as threading a needle or using a computer mouse. Flexibility of the hands allows for better hand-eye coordination, enabling individuals to perform tasks more accurately and efficiently.

4. Rehabilitation and injury prevention: Maintaining and improving hand dexterity, including flexibility, is important in injury prevention and rehabilitation. Flexibility exercises and stretching can help prevent conditions like carpal tunnel syndrome and tendonitis, which are common among individuals who perform repetitive hand movements. Additionally, flexibility exercises can aid in the recovery and rehabilitation of hand injuries or surgeries.

In conclusion, dexterity of the hands refers to the flexibility of the hands, allowing for a wide range of motion and precise movements. It is a crucial aspect of hand function and plays a significant role in performing various tasks that require fine motor skills.