:
Black has no other choice than to cut White's upper partial group apart.

:
Black has got two ko-threats.

A Black stone at (six liberties for White) would not have any effect, neither an additional one at (five liberties for White).

///:

:
Attacking White's lower stones, instead, is no valid ko-threat.

:
In the problem, White would connect the ko, instead.

:
Black gets some territory in the lower left.

:
But White is alive.