Final answer:
A sarcomere is defined by the region between two Z-lines, containing a central A band with overlapping actin and myosin filaments, and I bands with just actin filaments. When muscles contract, the H zone and I bands shorten, but the A band remains the same length.
Step-by-step explanation:
The correct answer to the student's question about which region describes a sarcomere is Region between two Z-lines. The sarcomere is the basic functional unit of a skeletal muscle fiber, and it spans from one Z line to the next Z line. The sarcomere contains parts of the muscle's contractile filaments, namely the thin filaments (actin) and thick filaments (myosin).
Detailed structure of a sarcomere includes the Z line, which is the boundary of each sarcomere, the I band, which consists of only thin filaments, and the A band, which is made up of overlapping thick and thin filaments. The H zone, present in the center of the A band, only has thick filaments and shortens when the muscle contracts. The sliding filament model explains how, during contraction, the thin filaments slide past the thick filaments, causing the Z lines to move closer together and shorten the entire sarcomere, but the A band's length remains unchanged.