Final answer:
The protein filament that slides toward the center of a sarcomere during muscle contraction is actin. This occurs as part of the sliding filament theory, where actin filaments move over myosin filaments to shorten the sarcomere and produce a contraction. The correct option is A).
Step-by-step explanation:
The protein filament that slides inward toward the middle of a sarcomere during muscle contraction is the actin filament. In the sliding filament theory, the thin filaments composed of the protein actin, along with its regulatory proteins troponin and tropomyosin, slide over the thick filaments, which are composed of the protein myosin.
In a relaxed muscle state, the myosin-binding sites on actin are blocked by tropomyosin. When Ca2+ ions bind to troponin, this causes tropomyosin to change shape and expose the myosin-binding sites.
The myosin heads then attach to these exposed binding sites on the actin filament and pull it towards the center of the sarcomere, leading to muscle contraction. Option A) is the correct one.