Thanks to Taxonomy_Single_Term, a library created by WebDevStudios, it made my work much easier on creating this plugin.

With Categories Metabox Enhanced, you can:

• Change the built-in Categories metabox/panel to a single term UI, which means replacing the checkboxes with radio buttons or a select drop-down.
• Apply the single term UI to other hierarchical taxonomies in the plugin’s Settings page.
• Customized the single term UI by setting these options:
  • Priority and position of the metabox placement.
  • Title of the metabox.
  • If child-terms should be indenting.
  • If adding of new terms from the metabox is enable.