A major and immediate effect of the situation where more than half of the U.S. population was living below the poverty line by the start of 1929 would be:
C consumer goods were not being sold.
This option reflects the immediate economic impact of widespread poverty, where consumers lack the financial means to purchase goods, ultimately affecting the economy and leading to further economic downturn. While other options are also relevant in the broader context of the Great Depression, the immediate effect of poverty would primarily be the lack of consumer spending.