B is the correct answer. You can't be certain if I, J, or K will be printed. Here's why...
J is obvious. There is no lock protecting it, hence, anything could happen.
I is less obvious. There is a lock protecting it, but there is a point between the two operations where there is no lock.
K is really subtle. You would think it is safe since there is a lock and it protects both operations. However, the check() method is also accessing the K variable, and without the protection of any lock.
In fact, you can just use the last reason, to say that you can't be certain of I, J, and K.