Pull the list of categories associated with a post and display the first one only.

Single Category Display

<?php 
/* Useful code snippet if you want to display the category title on top
 * of a featured image on a list of post. 
 * If a post is in more than one category, this snippet
 * selects the first one. If no cateogry is set, the default category appears. 
 */
 
		$post_categories = wp_get_post_categories( $post->ID );
		// get first category id 
		$first_cat = get_category($post_categories[0]); 
		// get category term info. 
		$term_link = get_term_link( $first_cat->slug, $first_cat->taxonomy );
 
		echo "<a href='".$term_link."'>".$first_cat->name."</a>"; 
?>