if you talk about frontend, than place your code inside a if-loop like that:
<?php
$custom_format = get_field('custom_format');
if($custom_format == "option2"){
if( get_field ('custom_infotext') ) : ?>;
<p class="custominfo">
<?php the_field('custom_infotext'); ?>
</p>
<?php endif;
}?>
replace custom_format with your fieldname, and option2 with the value of option2
