Slow oxidative fibers are suited for endurance-type activities.

Choice A reason: Perimysium surrounds bundles of muscle fibers known as fascicles, not individual muscle cells. It provides structural support and carries blood vessels and nerves.

Choice B reason: Epimysium is the outermost layer of connective tissue that surrounds the entire muscle. It does not envelop individual muscle cells.

Choice C reason: Endomysium is the correct answer. It is a thin layer of connective tissue that surrounds each individual muscle fiber (cell), providing structural integrity and facilitating capillary exchange.

Choice D reason: Myofibrils are intracellular structures within muscle cells that contain the contractile proteins actin and myosin. They are not connective tissue and do not surround muscle cells.

Choice A reason: Actin is a structural protein that forms the thin filaments in muscle fibers. It interacts with myosin during contraction but does not bind calcium directly or initiate contraction.

Choice B reason: Tropomyosin is a regulatory protein that blocks the myosin-binding sites on actin in a relaxed muscle. It shifts position when calcium binds to troponin, but it does not itself bind calcium.

Choice C reason: Titin is a large elastic protein that helps maintain the structural integrity of the sarcomere and contributes to passive elasticity. It does not function as a calcium receptor.

Choice D reason: Troponin is the correct answer. It is a regulatory protein complex that binds calcium ions during muscle contraction. This binding causes a conformational change that moves tropomyosin away from actin’s binding sites, allowing myosin to interact with actin and initiate contraction.

Choice E reason: Dystrophin is a structural protein that connects the cytoskeleton of muscle fibers to the extracellular matrix. It plays a role in muscle integrity but does not bind calcium or regulate contraction.