Which component of the bacterial structure is involved in muscle contraction?

Flagella are the correct choice because they play a critical role in bacterial motility. They are long, whip-like structures that extend from the surface of bacteria and enable them to move by rotating. While not directly involved in muscle contraction as seen in animal cells, flagella allow bacteria to move towards or away from stimuli, facilitating rapid response and adaptation in their environments. The mechanism of action in flagella involves the rotation of the basal body, which operates similarly to a motor, enabling the bacterium to 'swim'.

The other options do not pertain to muscle contraction. Endotoxins are components of the bacterial cell wall that can trigger inflammatory responses in hosts but do not have a role in movement. Pili are short, hair-like projections used for attachment or conjugation between bacteria, hence they do not contribute to muscle movement. Ribosomes are cellular structures involved in protein synthesis, essential for growth and division but unrelated to locomotion or muscle contraction.