How to put the author of the post in the comments?

by

Not sure it’s the shortest way, but it works)

Get post author id

$post_author_id = get_post_field( 'post_author', $c->comment_post_ID );

Get author data by id

$post_author_display_name = get_the_author_meta( 'display_name', $post_author_id );

Try this shorter version 

$output.= get_the_author_meta( 'display_name', get_post_field( 'post_author', $c->comment_post_ID ) );

Check documentation page which data is available with get_the_author_meta()