Please find the best result for black!
(japanese rules)
(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[4]KM[0.00]
PW[Weiß]PB[Schwarz]AW[ba][da][ab][bc]AB[dc][bd][cd]C[Black to play - what's the best he can do?]
(;B[cb]CR[cb]
(;W[bb]CR[bb]
(;B[ca]CR[ca]
;W[cc]CR[cc]
;B[db]CR[db]C[Seki, Black wins by one point (the prisoner) RIGHT])
(;B[cc]CR[cc]
;W[ca]CR[ca]C[Seki & Jigo....]))
(;W[cc]CR[cc]
(;B[ca]CR[ca]
;W[bb]CR[bb]
;B[db]CR[db]C[Seki, Black wins by one point (the prisoner) ])
(;B[bb]CR[bb]
;W[ac]CR[ac]
;B[db]CR[db]C[Black wins ])
(;B[db]CR[db]
;W[ca]CR[ca]C[This time, WHITE has an eye, and black has no one...]))
(;W[ca]CR[ca]
;B[bb]CR[bb]C[Black wins (one eye beats no eye)]))
(;B[cc]CR[cc]
;W[cb]CR[cb]C[Black missed the vital point]))