Muscle tone can best be described as:

Muscle tone refers to the continuous and passive partial contraction of the muscles, which keeps them ready for action. It is characterized by a slight tension that exists within the muscle fibers even when the muscle is at rest. This tension is provided by the ongoing activity of the muscle fibers, which ensures that the muscles maintain a certain level of firmness and readiness to respond to stimuli.

This readiness is crucial for maintaining posture and providing stability for movements, even when the muscles are not actively contracting. In contrast, the other options describe different states or characteristics of muscle function that do not accurately capture the essence of muscle tone. For instance, the idea of all fibers being active at once does not reflect the nature of muscle tone, as not all fibers need to be engaged simultaneously to provide tone. Similarly, relaxation of muscle fibers negates the concept of tone, which inherently involves a baseline level of tension.