Final answer:
Myosin II associates with actin fibers to form filaments that 'walk' on actin fibers in muscles and are responsible for muscle contraction.
Step-by-step explanation:
E. myosin II
In muscle cells, myosin II associates with actin fibers to form filaments that 'walk' on the actin fibers in a process called muscle contraction.
Myosin II is a motor protein that interacts with actin to slide the actin filaments past each other, causing muscle contraction.