How does Jane Austen show a preference for strong females in Pride and Predjudice?

This is a fascinating question, made all the more
interesting because I disagree with it. I assume that you would explain "preference" by
saying that the text makes clear Austen thinks favourably about the strong female
characters in this book. Whilst I think you are perfectly correct about Elizabeth Bennet
and the way that the author obviously shows a preference for her, I would disagree with
the presentation of other strong female characters, most notably Miss Caroline Bingley
and Lady Catherine de Bourgh, who are clearly strong female characters but also not
presented favourably in the text. Consider, for example, the way she tries to turn Mr.
Darcy against Lizzie and thereby advance her own
position:

She
often tried to provoke Darcy into disliking her guest by talking of their supposed
marriage, and planning his happiness in such an
alliance.

Clearly such mean
behaviour does not indicate that a character such as Caroline Bingley has a firm place
in the affections of Jane Austen. In addition, consider the way that Lady Catherine de
Bourgh takes Lizzie to task for the supposed alliance that exists between her and her
nephew. Both of these strong ladies are therefore not shown in the most flattering of
lights. Lastly, you might like to consider the way in which female characters who are
not strong are presented in a better light than these strong female characters, such as
Jane and Georgiana Darcy.