How to show one full post only on the category page

In my blog I have one post in each category. There are three categories with one post each.

The theme I’m using shows categories in a navigation menu by using
<?php wp_list_categories(‘title_li=’); ?>

However, the link that is created is to the category page which shows part of the one and only post.

How can I show the full post?

Thanks in advance.

Pali