The correct answer is - Birth rates declined during the Depression because families could not afford to care for infants and children.
The Great Depression is one of the worst, if not the worst, that has happened to the United States, from economic point of view. Lots of people lost their jobs, and lots of people became homeless, while the majority of the population was receiving much lower salaries.
The terrible financial situation result in declining birth rates, as people were finding it hard to survive on their own, yet alone if they had infants and children with them, and no parent wants the child to not be fed properly and to suffer.