A composition by Vladimir Korolkov

Great endgame study! Note that "9. ...Kxa8?" is not an error and deserves no question mark even though it is checkmate in 1 move. Endgame studies are never about delaying checkmate, always about forcing your opponent to find the only winning moves. There are two black moves which achieve that here and they are 9. ..Kxa8 and 9. ..a7-a6. They are called variations and neither is an error. All other black defenses permit white more than one winning move and are considered irrelevant.

If you wish to limit publication space and chose just one line of defense, which variation would you keep? Well, of course 9. ..Kxa8! and the exclamation mark is well deserved as the move brings out the idea of the whole study and ideas are the reason for creating a puzzle, not moves!