If Black loses the ko, the White connects at , creating a dead shape and thereby kills the entire Black group.
Even if Black gets to play the stones to maximize the eye space, Black is still dead. If Black actually played at a, then Black dies with a farmer's hat dead shape. Otherwise, White can almost fill the Black group with a bulky five dead shape by playing at a.
If , then
kills. See the analysis of ko section above for the reason.
at a or b is killed similarly.