Solution  

a,b,and c are the stronger responses to .

Resistance 'a'

 

If escapes, the remaining space isn't enough to create a seki.

continue  

captures the black stones by snapback.

Resistance 'b'

 

After , Black can't get both x and y

Resistance 'c'

 

After , White has x to escape or y to capture some black stones by snapback.

Capturing with only delays the problem; at is good enough.

The purpose of is to prevent the white chain with from escaping. If at x, still works.

 

Compare with Resistance 'c' the extra stone at allows Black to fight a ko.

 

Black captures everything.

 

For other , suffices.