Correct Option (d) Part between two Z-lines
Explanation:
Huxley reported the fine structure of striated muscle fibre or myofibril. Each myofibril is made up of A-bands (dark band anisotropic) and I-bands = isotropic (light bands isotropic). These two bands are made up of myosin and actin filament respectively. Each I-band is divided into two equal halves by a thin, fibrous and transverse zig-zag partition, called Z-band or Z-disc or Krause's membrane.
The part of the myofibril between two successive Z-lines functions as a contractile unit called sarcomere.
