The event that had the most influence on changing the ideology of the Democratic Party is B) the Great Depression occurring under Democratic President Franklin Delano Roosevelt.
The Great Depression led to significant shifts in American politics and the Democratic Party's ideology, particularly with Roosevelt's New Deal policies, which expanded the role of the federal government in the economy and introduced social welfare programs. This period realigned the party to better represent the working class and marginalized communities, setting the tone for modern liberalism within the party.