WordPress 3.0 comes with a new filter that lets us customize what is used for each permalink structure tag, other than WordPress default.
One of these permalink structure tags is <code>%category%</code>. By default, WordPress always use the category with lowest ID, making <code>%category%</code> impractical for SEO optimization.
Hikari Category Permalink allows post authors to choose among each post’s categories, which of them is used in that post permalink, giving much more flexibility and power to permalinks.
This plugin is a fork of <a href="https://wordpress.org/extend/plugins/scategory-permalink/" rel="ugc">Dmytro’s sCategory Permalink</a>. It has all original features and is compatible with original options, while being more stable and simple, and also fixes 2 recurring bugs.
<h4>Features</h4>
<ul>
<li>You can choose for each post separately, which category is used in its <code>%category%</code> permalink.</li>
<li>Posts without a category set to be used in permalink behave as WordPress default, (which currently is) the one with lowest ID is used</li>
<li>Posts with a category set have it used</li>
<li>In post edit page, where you set the post’s categories, there’s a new feature allowing you to choose which category will be used in permalink</li>
<li>No rewrite rules tweaks are done, making the plugin much simpler and bug free</li>
<li>If you already used sCategory Permalink, you can safely deactivate it and replace by Hikari Category Permalink, all your category permalinks will be used</li>
</ul>

Hikari Category Permalink

For each post, author can choose which category is used in permalink.

Dmytro Shteflyuk

Permalink option of WordPress <code>%category%</code> has one great limitation – 
when this option is selected, WordPress uses category with lowest ID for 
permalink generation. This plugin is intented to bypass WordPress permalinks 
limitation and allows you to select category for permalink generation.
<h4>Support</h4>
If you have any suggestions, found a bug, or just wanted to say “thank 
you”,– feel free to email me <a href="mailto:kpumuk@kpumuk.info" rel="nofollow ugc">kpumuk@kpumuk.info</a>. 
Promise, I will answer every email I received.
If you want to contribute your code, see the Development section under 
the Other Notes tab.
<h3>Development</h3>
Sources of this plugin are available both in SVN and Git:
<ul>
<li><a href="http://svn.wp-plugins.org/scategory-permalink/" rel="nofollow ugc">WordPress SVN repository</a></li>
<li><a href="http://github.com/kpumuk/scategory-permalink/" rel="nofollow ugc">GitHub</a></li>
</ul>
Feel free to check them out, make your changes and send me patches. 
Promise, I will apply every patch (of course, if they add a value to the 
product). Email for patches, suggestions, or bug reports: 
<a href="mailto:kpumuk@kpumuk.info" rel="nofollow ugc">kpumuk@kpumuk.info</a>.

sCategory Permalink

Plugin allows to select category which will be used to generate permalink on post edit page. Use custom permalink option %scategory%.

Hikari Category Permalink vs sCategory Permalink