Final answer:
Myosin cross-bridges attach to the active sites on actin filaments (option A) during muscle contraction, facilitating the movement necessary for the contraction of skeletal muscles.
Step-by-step explanation:
During muscle contraction, myosin cross bridges attach to the active sites on actin filaments. This attachment is crucial for the contraction process of skeletal muscles. The cycle begins when calcium ions (Ca2+) bind to troponin, which causes a conformational shift in tropomyosin, revealing the myosin-binding sites on actin. The myosin heads, with attached ADP and phosphate, then bind to these exposed sites on the actin filaments, forming cross-bridges.
Next, a power stroke occurs as the myosin heads pivot, pulling the actin filaments towards the center of the sarcomere, and the attached ADP and phosphate are released. ATP then binds to the myosin heads, causing them to detach from the actin, and upon hydrolysis of ATP, the myosin heads are re-cocked for another cycle of attachment and movement.
Thus, the correct answer to the student's question is: A - Actin filaments.