This code is echoing out an entire image tag
<?php echo wp_get_attachment_image( $hero, $size ); ?>
What you want to do would be more like
<?php
$image = wp_get_attachment_image_src($hero, $size);
?>
<div class="c-1 hero bg-image" style="background-image: url('<?php
echo image[0]; ?>');">
...
</div>
