Final answer:
The letter associated with the dark bands in a sarcomere is A, referring to the A band, which includes overlapping thick and thin filaments and appears darker.
Step-by-step explanation:
The letter associated with the dark bands in a sarcomere is A, which stands for the A band. The A band at the center of the sarcomere contains overlapping actin and myosin filaments, making it appear darker under a microscope. During a muscle contraction, the A band's width does not change, distinguishing it from other bands such as the I band, which gets smaller. The H zone is the central region of the A band that contains only thick myosin filaments and shortens during contraction. The I band is lighter and contains only thin actin filaments, and the Z line, or Z disc, marks the boundaries of a sarcomere.