What is AS path prepending?

when a router, sending a BGP update, adds the AS number of the router from which it received the route to the AS$-$path attribute

when a router, sending a BGP update, adds the AS number of the router to which it is sending the route to the AS$-$path attribute

when a router, sending a BGP update, adds its AS number to the AS$-$path attribute multiple times

when a router uses the AS$-$path attribute during route selection