Hi M,
As a starter, here are two prior discussions of this particular drill item that you might find helpful (just in case):
More specifically to your question, we know from the setup that only D, F, K, and L can possibly perform on Thursday on the East stage. The question stem schedules D to perform on the west stage, leaving only F, K, and L for Thursday on the East stage. From the 4th rule, we can also rule K out from performing on Thursday on the East stage, leaving only F and L as possibilities for Thursday's East stage performance.
However, the question stem also schedules N for Saturday, and because of the effects of the first rule, this means that L must perform on Friday. With L now removed from Thursday, that leaves only F remaining to perform on Thursday on the East stage, and thus answer choice (B) must be true and is correct.
You mention K performing on Thursday; I think you meant K performing on Thursday on the East stage, but that is not possible since D must perform on Thursday per the question stem, and the 4th rule states that "D and K cannot perform on the same day."
Please let me know if that helps. Thanks!