Final answer:
Fetal sleep states usually do not last longer than a. 30 minutes, corresponding to answer choice a. 30. This stage of development includes many physiological changes, but is not directly related to sleep cycle lengths.
Step-by-step explanation:
Fetus sleep states do not usually last longer than 30 minutes. Therefore, the correct answer is a. 30. During developmental weeks 21-30, the fetus undergoes significant physical and developmental changes, including rapid weight gain essential for maintaining a stable body temperature after birth, the beginning of myelination of the axons of the spinal cord, growth of eyelashes, and the ability to open and close the eyelids. Additionally, surfactant production in the lungs commences, vital for lung function post-birth. Although these developmental milestones are fascinating, they're separate from sleep cycles but show the complexity of fetal development during this period.