Questions: A power stroke involves an actin molecule pulling a myosin molecule toward the M-line. a myosin head pulling a thin filament toward the center of the sarcomere. a thick filament pushing an actin filament towards the nearest I band. a thin filament shortening in length and thereby shortening the sarcomere.

The answer is the second one: a myosin head pulling a thin filament toward the center of the sarcomere.

Explanation for each option:

1. An actin molecule pulling a myosin molecule toward the M-line.
   This is incorrect. In muscle contraction, it is the myosin heads that pull the actin filaments, not the other way around.

2. A myosin head pulling a thin filament toward the center of the sarcomere.
   This is correct. During a power stroke, the myosin head binds to actin and pulls the thin filament (actin) toward the center of the sarcomere, which is the fundamental process of muscle contraction.

3. A thick filament pushing an actin filament towards the nearest I-band.
   This is incorrect. The thick filament (myosin) does not push; it pulls the actin filament toward the center of the sarcomere, not towards the I-band.

4. A thin filament shortening in length and thereby shortening the sarcomere.
   This is incorrect. The thin filament does not shorten in length; instead, it slides over the thick filament, which results in the shortening of the sarcomere.

In summary, the power stroke involves the myosin head pulling the thin filament toward the center of the sarcomere, facilitating muscle contraction.