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<?php

// get all product cats for the current post
$categories = get_the_terms( get_the_ID(), 'product_cat' ); 

// wrapper to hide any errors from top level categories or products without category
if ( $categories && ! is_wp_error( $category ) ) : 

// loop through each cat
foreach($categories as $category) :
// get the children (if any) of the current cat
$children = get_categories( array ('taxonomy' => 'product_cat', 'parent' => $category->term_id ));

if ( count($children) == 0 ) {
// if no children, then echo the category name.
echo $category->name;
}
endforeach;

endif;


?>