answer
The correct answer is D.
A sarcomere extends from one Z-line to another Z-line, and consists of thick filaments (myosin), thin filaments (actin), and sarcomeric proteins. Thick filaments (and sarcomeric proteins) are primarily responsible for maintaining the structural integrity of the sarcomere-areas in the sarcomere which consist only of thin filaments are more susceptible to structural compromise.
The I band consists only of actin (thin filaments) ∴ the I band is the most common site for muscle strains.
The A band consists of myosin (thick filaments) and actin (thin filaments). On electron microscopy:
1) The A band has darker peripheral regions in which actin and myosin filaments overlap and myosin heads are located.
2) The A band also has a lighter central region called theH band, which is situated between the free ends of thin (actin) filaments on either side and contains only the shafts (not the heads) of myosin polypeptides.
The M-line bisects the H band and represents the center of the sarcomere. The M-line holds the myosin (thick) filaments in place.