A. By the end of the nineteenth century, most monarchies in Europe and North America were replaced by democracies.
Not supported,
first, we can't infer about
most from
many and
second, we don't whether monarchies were replaced by democracies or something else.
B. Most countries which were monarchic in Europe and North America at the beginning of the 18th century had a different form of government by the end of the 19th century.
Not supported, we can't infer about
most from
many.
C. Though accepted in the twentieth century by certain countries, communism has philosophical and social roots dating to the 18th and 19th centuries.
Not supported, no such info given in the argument.
D. There were many reasons for the regime changes of the 18th and 19th centuries.
Not supported, we are
not given about many reasons for regime changes.
E. You were more likely to live in a monarchy on the eve of the 18th century than you were on the eve of the 20th century.
Supported, there were more number of monarchies in 18/19th century than 20th century so it will be more likely to live in a monarchy in former as compared to latter.