This problem is adapted from a pro game. Won Sungjin found a nice tesuji near the end of the game and Park Yenghun had to resign. Kill the marked stones. You can win any ko! Please comment if there's anything important missing and I'll add more variations.
(;AB[cc]AB[cd]AB[de]AB[df]AB[cg]AB[ch]AB[bi]AB[bh]AB[bj]AW[aj]AW[bk]AW[ck]AW[dk]AW[cj]AW[ci]AW[dj]AW[ei]AW[fi]AW[dh]AW[dg]AW[ef]AW[gf]AW[fe]AW[fd]AW[dc]AW[eb]AW[fb]AW[cb]AW[bb]AB[bl]AB[cl]AB[bn]AB[cn]AB[en]AB[em]AB[fm]AB[ek]AB[fk]AB[gk]AW[gc]AW[ga]TR[df]TR[de]TR[cd]TR[cc]C[White to kill the marked stones.]AP[goproblems]
(;W[bf];B[be]
(;W[cf]
(;B[af]C[CHOICE]
(;W[ai];B[ah]
(;W[ag];B[ak]
(;W[ae];B[ce]
(;W[aj]C[White prevents Black from approaching and gets a ko for the marked stones.RIGHT]TR[cc]TR[cd]TR[be]TR[ce]TR[de]TR[df];B[ad]
(;W[ac];B[af]C[Great! White gets a ko! It's a three step ko but it decided the game as Black shortly resigned.RIGHT])
(;W[bc]C[RIGHT];B[af]C[RIGHT]))
(;W[ad];B[bg];W[af];B[ac]))
(;W[aj];B[bg]))
(;W[ak];B[ce];W[bg];B[ag];W[bf];B[ad]))
(;W[ag];B[ae]
(;W[ai];B[ce];W[ah];B[bg];W[ak];B[bc])
(;W[bg];B[ce];W[ai];B[ah])
(;W[ah];B[ce];W[ai];B[bg];W[ak];B[bc]))
(;W[ad];B[ai]
(;W[ak];B[ag])
(;W[ag];B[ae]
(;W[ah];B[bg];W[ag];B[ce])
(;W[bg];B[ak];W[bd];B[ce])
(;W[bd];B[ce];W[bg];B[ah])
(;W[ak];B[ce];W[bg];B[ah])))
(;W[bc];B[ai]
(;W[ak];B[ag])
(;W[ag];B[ae];W[bg];B[ak])
(;W[bg];B[ak];W[ah];B[ag])))
(;B[ag]C[RIGHT];W[ai];B[ad];W[bc]
(;B[af];W[bd]C[RIGHT])
(;B[bd];W[af]C[RIGHT])))
(;W[bc];B[cf]
(;W[bd];B[ai]
(;W[ak];B[af])
(;W[ae];B[ak]))
(;W[ai];B[bd])
(;W[af];B[ai]
(;W[ak];B[ag];W[ae];B[ad];W[bd];B[bg])
(;W[ae];B[ak])
(;W[ag];B[ae];W[ak];B[bd];W[dd];B[ah];W[ac];B[bg])))
(;W[ag];B[cf];W[ae];B[ad];W[bc];B[bd];W[ai];B[ah];W[ak];B[bg])
(;W[ae];B[ad];W[ag];B[cf];W[bc];B[bd];W[ai];B[ah];W[ak];B[bg]))
(;W[ae];B[bf];W[be];B[ce]
(;W[ag];B[ai];W[ak];B[ad];W[bd];B[bc];W[ac];B[af];W[ab];B[ad])
(;W[ai];B[ah];W[ak];B[ad];W[bd];B[bc];W[ac];B[af];W[ab];B[ad]))
(;W[cf];B[bf];W[be];B[bd];W[af];B[ce];W[ad];B[ac];W[bc];B[ae])
(;W[be];B[bf]
(;W[ce];B[bd];W[ad];B[ac];W[af];B[cf];W[bc];B[ae];W[ai];B[ah])
(;W[ad];B[ce]
(;W[ai];B[ah];W[ak];B[bd];W[af];B[ac];W[bc];B[ae])
(;W[af];B[ai];W[ak];B[ag]))
(;W[bd];B[ce]
(;W[ag];B[ai];W[ak];B[bc];W[ac];B[ae];W[ab];B[ad])
(;W[ai];B[bc];W[ac];B[ae];W[ab];B[ad])
(;W[bc];B[ai];W[ak];B[ag]))
(;W[ag];B[bc];W[ad];B[ac];W[af];B[ai];W[ak]
(;B[ce])
(;B[bd];W[bg];B[cf];W[dd];B[ce]))))