Upon contraction of sarcomere in skeletal muscle fibres, reduction is not observed in
Length of actin filaments
Size of region enclosed by two Z lines
The correct option is A Length of actin filaments Sarcomeres are composed of long, fibrous proteins as filaments that slide past each other when a muscle contractor relaxes. Two of the important proteins are myosin, which forms the thick filament, and actin, which forms the thin filament. Upon muscle contraction, the A-bands do not change their length (1.85 micrometers in mammalian skeletal muscle), whereas the I-bands and the H-zone shorten. This causes the Z lines to come closer together.
So the correct option is 'Length of actin filament'.