Structure of Striated Muscle:
- Striated muscle is composed of repeating units called sarcomeres, which are the functional units responsible for muscle contraction.
- Sarcomeres are made up of thick and thin filaments that slide past each other during muscle contraction.

Components of Sarcomere:
- Z-band: The Z-band is a protein structure that serves as the attachment site for actin filaments. It marks the boundary of each sarcomere.
- I-band: The I-band contains only thin filaments and is located between two adjacent sarcomeres.
- A-band: The A-band contains both thick and thin filaments and is located at the center of the sarcomere.
- H-zone: The H-zone is a region within the A-band that contains only thick filaments.

Function of Sarcomere:
- During muscle contraction, the thin filaments slide past the thick filaments, resulting in the shortening of the sarcomere.
- This sliding of filaments is facilitated by the interaction of myosin heads (thick filaments) with actin filaments (thin filaments) through cross-bridges.

Role of Sacromere:
- The sarcomere is the basic contractile unit of muscle that generates force and movement by shortening during contraction.
- It is within the sarcomere that the sliding filament theory of muscle contraction occurs, where actin and myosin filaments interact to produce muscle movement.
- The Z-band marks the boundary of each sarcomere and plays a crucial role in maintaining the structural integrity of the muscle fiber.
Therefore, the correct answer to the question is the sarcomere, as it is the fundamental unit of the contractile system in striated muscle responsible for muscle contraction.