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.

Hikari Category Permalink vs Fix Error 404 permalinks