What Defines What Category A Post Picks (if in multiple)

WordPress always use the category with lowest ID. You can use plugins such as the Hikari Category Permalink plugin to change that however.

http://wordpress.org/extend/plugins/hikari-category-permalink/

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)