Answer:
Separation between sarcomeres.
Step-by-step explanation:
The structure of the muscle fibre is composed of many functional units called sarcomere which is interrupted by the discs called Z-discs.
These Z-discs are made of the alpha-actin homodimers which allow the actin filament allows to bind to it along with other proteins. The Z-discs since present at regular intervals in a muscle appears to separate the sarcomere and defines the boundaries of the sarcomere.
Thus, the selected option is correct.