The best explanation for why the Great Depression spread to Britain is:
a dramatic drop in American purchases of luxury items and overproduction.
This option highlights the interconnectedness of the global economy at the time, particularly the impact of the U.S. economy on other countries, including Britain. When American consumers reduced their spending and overproduction occurred, it led to economic challenges not only in the U.S. but also in Britain and other nations reliant on American goods and investment.