The correct statement for muscle contraction is :

Option A: Length of H-zone decreases During muscle contraction, the sliding filament theory explains that the thin filaments (actin) slide over the thick filaments (myosin) toward the center of each sarcomere. This results in a decrease in the length of the H-zone, which is the region in the center of the sarcomere that is primarily composed of myosin and devoid of actin. To present the information more clearly: - Option A: Length of H-zone decreases - Correct.

- Option B: Length of A-band remains constant - This is also correct; however, it is not the choice that is being asked for confirmation. The A-band length, which corresponds to the length of the myosin filaments, indeed remains constant during contraction.

- Option C: Length of I-band increases - This is incorrect. The I-band, which contains thin filaments with no overlapping thick filaments, actually decreases during muscle contraction.

- Option D: Length of two Z-line increases - This is incorrect. The distance between two Z-lines, or the length of the sarcomere, decreases during muscle contraction as the Z-lines move closer together. The H-zone's change in length is a direct indicator of muscle contraction because it specifically refers to the action of the sliding filaments during the process.