The correct answer is option 'B', which states that the contractile unit of muscle is a part of the myofibril between the Z line and Z line. Let's understand this in detail.

Myofibrils and Muscle Contraction:
Muscles are made up of smaller structures called muscle fibers, which in turn are composed of myofibrils. Myofibrils are thread-like structures that are responsible for muscle contraction. They consist of repeating units called sarcomeres, which are the functional units of muscle contraction.

Sarcomeres:
Sarcomeres are the basic contractile units of skeletal muscle and are responsible for the generation of force during muscle contraction. They are located between two Z lines and are aligned end to end along the length of the myofibril.

Components of Sarcomeres:
Sarcomeres are made up of several components, including thick filaments (composed of the protein myosin), thin filaments (composed of the protein actin), and various regulatory proteins. These filaments are arranged in a highly organized manner within the sarcomere.

Z Line:
The Z line (also known as the Z disc) is a protein structure that serves as an anchor for the thin filaments and provides structural support for the sarcomere. It marks the boundaries of the sarcomere and is responsible for maintaining the alignment and stability of the myofibrils.

Hence, the Z line defines the ends of the sarcomere and separates one sarcomere from the next.

Contractile Unit of Muscle:
The contractile unit of muscle is the smallest functional unit within a muscle fiber that can contract. It corresponds to a single sarcomere, which is the region between two Z lines.

During muscle contraction, the thick and thin filaments slide past each other, causing the sarcomere to shorten. This sliding filament mechanism is responsible for the overall shortening of the muscle fiber and the generation of force.

Conclusion:
In conclusion, the contractile unit of muscle is a part of the myofibril between the Z line and Z line. This region corresponds to a single sarcomere, which is the functional unit of muscle contraction. The Z line marks the boundaries of the sarcomere and provides structural support for the myofibrils. Understanding the organization and function of sarcomeres is crucial for comprehending the mechanism of muscle contraction.