What is the smallest contractile unit of a muscle cell?

A myofibril is a cylindrical organelle composed of many sarcomeres arranged in series, making it a larger organizational unit rather than the smallest contractile unit.

A sarcomere is the repeating unit of a myofibril delimited by two Z-lines, and it is the smallest segment of muscle tissue capable of independent contraction. It contains thick myosin filaments and thin actin filaments whose ATP-driven cross-bridge cycling produces the sliding filament action that underlies all muscle force generation.

Myosin is the motor protein comprising the thick filament within a sarcomere - it is a molecular component of the contractile apparatus, not the contractile unit itself.

The sarcolemma is the plasma membrane surrounding the muscle fiber that propagates action potentials; it is an electrical signaling structure, not a contractile unit.