The idea that there should be a high-degree mobility from one class to another would be acceptable to A. Socialists and Capitalists B....

This is really going to depend on how your particular text
or teacher defines all of these ideologies.  However, I would argue that the best answer
to this is B.

Liberalism is clearly the ideology that is
most positive about the idea of people moving from class to class.  Liberals believe
that people should be able to do and be whatever their abilities allow.  Conservatives,
on the other hand, are harder to pin down on this issue.  An American conservative would
certainly agree that people should be able to move from class to class as their
abilities and efforts warrant.  However, conservatism is also a "status quo" ideology
and some people might argue that conservatives would want the class hierarchy to remain
the same.

However, I would argue that B is the best of the
choices given.