John: I suggest adding to the description of the problem that it is a whole board position, explicitly. This will resolve all of the uncertainty. Otherwise it is unclear whether winning this ko is critical.
John: I agree with previous comments that B5 should be accepted as the correct move. It does loose points, but having more ko threats may be more important than loosing some points. The choice fully depends on the whole board position. The fact that the whole board is shown doesn't mean that there are no valid ko threats, it might be just a bug in the program. So, either both B5 and A4 ko threats should be correct, or it should be clearly stated that there are no big enough ko threats elsewhere.
Ivan Detkov: One shall not use point-losing ko-threats unless it is absolutely necessary to win a ko. Case closed.
Paul Thibodeau: Over 20 local points are at stake in the Ko, not to mention life for the outside stones, and the impact on the opponent's surrounding stones. Comments are confusing decisive middlegame Ko fights at the 20-40 point stage, with endgame plays at the 1-or-2 points stage.
posetcay: This is a whole board tsumego, which means white only needs 1 threat to win the ko. Therefore it is pointless to play point-losing threats.
Met: And you can lose because you lost the ko by one threat and everything, including the outside stones, dies. The choice of whether to play the 1 or 2 ko threats is situational and either can be correct in different game. Moreover, this is a beginner problem that should not fail on a relative detail.
Sadaharu: You can also lose the game because you lost points when using ko threats. Beginners shouldn't use ko threats that lose points. Also, without knowing the outside situation, the opponent has no local ko threats, so there is no need to increase ko threats by playing moves that lose points.
Met: Comment below. B5 threat should be accepted. Nothing says the points are more important than connecting.
I broke my accuracy streak at ~150 over this >:(
Ivan Detkov: Points are the most important thing in go. In a game you can lose because you played B5 instead of B4.
One shall not use point-losing ko-threats unless it is absolutely necessary to win a ko. Case closed.