How to call in alternate options set in the page with the WordPress plugin, Advanced Custom Fields (ACF)
<?php if( get_field('external_link') ): ?>
<a href="<?php the_field('external_link'); ?>" target="_blank"><?php the_post_thumbnail('img-fluid w-100'); ?></a>
<?php elseif( get_field('internal_link') ): ?>
<a href="<?php the_field('internal_link'); ?>"><?php the_post_thumbnail('img-fluid w-100'); ?></a>
<?php else: // field_name returned false ?>
<?php the_post_thumbnail('img-fluid w-100'); ?>
<?php endif; ?>