Show only one category from the the_category_list() function.

show-one-category.php

<?php
add_filter( 'the_category_list', function( $categories ) {
    if( ! empty( $categories ) && is_array($categories) ) {
        foreach ($categories as $key => $cat) {
            if( ! $cat->parent ) {
                $ob[] = $categories[$key];
                return $ob;
            }
        }
    }

    return $categories;
}, 100 );