Which of the following proteins is involved in muscle contraction?

The protein involved in muscle contraction is actin. Actin plays a critical role in the cellular structure and dynamics of muscle fibers. During muscle contraction, actin interacts with myosin, another key protein that forms thick filaments in muscle cells. This interaction, often described by the sliding filament model of muscle contraction, allows for the shortening of muscle fibers, leading to movement.

The presence of actin filaments allows for the formation of a contractile structure that is essential for the actual contraction mechanism in both skeletal and cardiac muscles. Actin provides a track for myosin heads to attach and pull, which results in the contraction process that facilitates muscle movement. Other proteins mentioned in the choices do not have a direct role in muscle contraction; for instance, keratin is a structural protein found in hair and nails, hemoglobin is associated with oxygen transport in the blood, and estrogen is a hormone involved in various biological functions but not directly in muscle contraction.