Which of these 19th century ideology emphasized the greatest degree of freedom of opinion and dissent? A. Liberalism B. Nationalism C. Communism...

The best answer to this question is
A.

Liberalism was an ideology that pretty closely resembled
the beliefs that we have in the United States today.  Liberalism is based very strongly
on the idea that individual people have rights and that the government should have to
honor those rights.  As the link below says, liberalism
is

... a set
of attitudes, including particularly an emphasis on the recognition of the rights of the
individual and tolerance, which permits considerable diversity of views among
liberals.

As you can see from
this quote, liberalism is quite clearly an ideology that is based more than anything
else on the idea that people should be able to express their points of view on various
issues.  Therefore, the best answer is A.