How to get a category in a list item class

I am need to know how would I get the category to be dynamically input in a WP_Query

This is the code I am using:

<li class="<?php the_category(' ')?>">

but the output is breaking.

This is all the code

 <ul id="portfolio-list">

<?php

$args = array(
    'post_type' => 'post',
    'post__and' => array( 15, 14, 17, 13, 10, 8, 12, 11, 9, 16),
);

$the_query = new WP_Query( $args );
?>

<?php if ( $the_query->have_posts() ) :?> 


    <?php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>

  <li class="<?php the_category(' '); ?>">


<?php the_post_thumbnail( $size, $attr ); ?> 

    <?php the_title(); ?>

    <br />

    <?php //the_category('cat-slug'); ?>

<?php endwhile; else: ?>

    <p>There are no posts or pages here</p>

<?php endif; ?>

<?php wp_reset_postdata(); ?>

    </li>

</ul>