Final answer:
The statement describing the relaxation phase of a sarcomere is 'The sarcomeres are increasing in length, and the force generated decreases'. The correct option is C.
Step-by-step explanation:
During relaxation, the muscle fiber returns to its resting state and the tension within the muscle decreases. This is a result of the sliding filament model of muscle contraction where, upon relaxation, the actin and myosin filaments within the sarcomere move apart, increasing the length of the sarcomere.
In contrast, during contraction, myosin heads bind to actin forming cross-bridges and pull the actin filaments toward the M line, which shortens the sarcomere and increases muscle tension. This contraction is powered by the hydrolysis of ATP.
In the relaxation phase, this process is reversed, and the sarcomeres return to their original length as the myosin heads release the actin filaments. Hence, C is the correct option.