Wouldn't black rather live by playing at b or c instead of inside his own territory?
unkx80: Good point here, but the emphasis here is that white has to do something inside black's territory before he can even think of killing.
joshsz: My first thought was to close off potential eyespace, but after looking at it some more I see that black can play at and create two eyes.
Jacob4Jesus: should be at a. As is, a and b make a false eye and at b kills.
MrTenuki: Actually, is enough because Black can respond to at b with at a. Essentially, White has one ko threat but nothing more. Now, playing at a is indeed another valid option, although White still gets one ko threat at .
Mankarr: I am slightly confused by the solution stating at the 1-3 point is the strongest resistance. Wouldn't at the 1-2 point here be the strongest? I think White still needs to play and after Black still has a ko threat under the stones. Is my thinking flawed? Is the previous solution the stronger resistance due to White's shape?