The thinker whose name is most clearly identified with political liberalism isThe thinker whose name is most clearly identified with political...

Since your question clearly specifies that you are asking
about political liberalism, the best answer to this
question is D.

The only possible answers here are A and D. 
Robert Owen is best known as a utopian socialist and is therefore not really a political
liberal in the classical sense (as described in the link below).  Engels was a Marxist
and is therefore clearly not interested in liberalism.

Of
the other two, Mill is the better answer than Smith.  This is because Smith was not
really particularly interested in politics.  Smith was certainly an economic liberal who
pushed for people to have broad economic rights.  Mill, however, is much more interested
in political liberalism.  This is why he is listed in the
link below as a major figure in liberalism.