Final answer:
The A band remains the same width during the active shortening of a striated muscle fiber, as described by the Sliding Filament Model of muscle contraction.
Step-by-step explanation:
When a striated muscle fiber actively shortens, the only band that remains the same is the A band. During muscle contraction according to the Sliding Filament Model, while the I band and H zone get smaller, the A band does not shorten but retains its length.
This is because the A band, which contains the thick filaments of myosin, is the region where myosin and actin (thin filaments) overlap, and the sliding action during contraction doesn't affect its length, only the amount of overlap changes. The Z lines, which define the borders of a sarcomere move closer together, and as the I bands get smaller, the whole muscle shortens.