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[ab][ac][dc]AB[da][db][ad][bd][cd]C[Black to play - what's the best he can do?]
(;B[cb]CR[cb]
;W[bb]CR[bb]
(;B[cc]CR[cc]
;W[ba]LB[dd:A]CR[ba]C[Please play A now...]
;B[dd]CR[dd]C[Seki, Black wins by one point (the prisoner) RIGHT])
(;B[bc]CR[bc]
;W[ba]CR[ba]C[Seki, Black wins by one point (the prisoner). RIGHT]))
(;B[cc]CR[cc]
;W[cb]CR[cb]
;B[bb]CR[bb]
;W[ba]CR[ba]C[White wins!])
(;B[bb]CR[bb]
;W[cc]CR[cc]
;B[bc]CR[bc]
;W[cb]CR[cb]
(;B[ba]CR[ba]
;W[ca]LB[aa:A]CR[ca]C[Please play A now.]
;B[aa]CR[aa]C[Seki & jigo (two prisoners each)])
(;B[aa]CR[aa]
;W[ca]CR[ca]
(;B[ba]CR[ba]C[Seki & jigo (2 prisoners each)])
(;B[dd]CR[dd]
;W[ba]CR[ba]C[White wins.])))
(;B[bc]CR[bc]
;W[cb]CR[cb]
;B[bb]CR[bb]
;W[cc]CR[cc]
;B[ba]CR[ba]
;W[ca]LB[aa:A]CR[ca]C[Please play A now.]
;B[aa]CR[aa]C[Seki & jigo (two prisoners each)]))