Explanation
During muscle contraction, the correct answer is A: Size of A-bands remains the same.
Muscle contraction occurs when the actin and myosin filaments in the sarcomere, the basic contractile unit of muscle, slide past each other, causing the sarcomere to shorten. The A-band, which contains the thick myosin filaments, remains the same size during muscle contraction as the myosin filaments do not change in length. The H-zone, which is the area in the center of the A-band where there are no overlapping thin actin filaments, becomes smaller as the actin filaments slide inward towards the center of the sarcomere. The I-band, which is the region between the A-band and the Z-line where there are only thin actin filaments, also decreases in size as the actin filaments slide inward towards the center of the sarcomere.
However, the diameter of the muscle fiber does not increase during muscle contraction. In fact, the diameter of the muscle fiber may actually decrease slightly during contraction due to the compression of the myofibrils within the fiber..
