There's no action that will be best every time. When you're on a short stack, you eventually have to pick a hand to risk all your chips on, and you don't have lots of time to wait. K9s is a bit better than average, so not a great hand, especially against a raiser who has put you all in, which means you have zero fold equity. However, if the raiser is loose and/or playing the table bully, you my even be ahead. And if you're not, you still have decent odds
if your cards are live.