Final answer:
The sliding filament theory is supported by the evidence that the H band decreases in width during muscle contraction, while the A band's width remains constant and the I band also decreases. Therefore, the correct option is (c) decreased width of the H band during contraction.
Step-by-step explanation:
The question pertains to the sliding filament theory of muscle contraction and is seeking evidence that supports this theory. According to the sliding filament theory, when a muscle cell contracts, several changes occur within the structure of the sarcomere, which is the functional unit of muscle fibers. Notably, the H band, which contains only the thick (myosin) filaments, decreases in width because the thick and thin (actin) filaments slide past each other and the sarcomere shortens. Conversely, the A band, containing the full length of the thick filaments, whether overlapped by thin filaments or not, remains at a constant width during contraction. The I band, containing only thin filaments, also gets smaller as the sarcomere shortens and the Z lines move closer together.
Out of the options presented, the correct option that serves as physical evidence for the sliding filament theory is (c) decreased width of the H band during contraction. This phenomenon aligns with the description of the changes observed in the sarcomere during muscle contraction.