The clamp of is the solution. The strongest resistance black can try is a; b creates too many cutting points, while c allows white to live in the corner.
After black connects, white connects under with and lives through a complicated running fight. Although black can try A or B, neither will kill white.
Next white tries to capture the three black stones. Notice is threatening to play at in sente and then make life with a. Were White to play at , she would have miai of playing at a and capturing the two marked Black stones. Black destroys White's eye with . After , Black can't escape.