The correct answer is option 'B' - eye muscles. Eye muscles are the busiest muscles in the human body. This is because our eyes are constantly moving and adjusting, even when we are not consciously aware of it.

Eye movements:
- Our eyes move in different directions to scan the environment, track moving objects, and focus on different points of interest.
- Eye movements are controlled by six different muscles that work together to move the eyes up and down, left and right, and diagonally.
- These muscles are constantly working to allow us to see and perceive the world around us.

Blinking:
- Blinking is another activity that keeps the eye muscles busy.
- We blink to keep our eyes moist and to protect them from dust and other particles.
- On average, a person blinks about 15-20 times per minute, which means our eye muscles are constantly contracting and relaxing.

Reading and focusing:
- When we read or focus on objects at different distances, our eye muscles are constantly adjusting the shape of the lens inside the eye to bring the object into clear focus.
- These constant adjustments require the eye muscles to work continuously.

Eye strain:
- The constant use of eye muscles can sometimes lead to eye strain or fatigue, especially when we spend long hours staring at screens or performing tasks that require intense visual concentration.
- Taking breaks, looking away from the screen, and practicing eye exercises can help reduce eye strain and keep the eye muscles healthy.

In conclusion, eye muscles are the busiest muscles in the human body because they are constantly moving, adjusting, and focusing to allow us to see and perceive the world around us.