Final answer:
The GI Bill most contributed to the educational and economic trends discussed in the excerpt by providing veterans with benefits such as college tuition, unemployment benefits, and low-interest loans, resulting in an increase in the middle class and the enrichment of U.S. society.
Step-by-step explanation:
The development that most contributed to the educational and economic trends discussed in the excerpt is the GI Bill. The GI Bill of Rights, passed by Congress to assist veterans in transitioning to civilian life after WWII, provided various benefits such as college tuition, unemployment benefits, and low-interest loans. This resulted in a substantial increase in the middle class and the enrichment of U.S. society through the education and training of veterans in various professions.