The sentence requires a phrase that introduces a contrasting idea, similar to "However" in Karen's original statement. "but in her opinion": This phrase introduces a contrasting viewpoint. Karen agrees the concert was amazing, but* she has a different opinion about their earlier music. This fits. "since she thinks": This phrase introduces a reason. It implies that her thinking their earlier music is best is the reason* the concert was amazing, which is incorrect. "because she is sure that": This phrase also introduces a reason. It implies that her certainty about their earlier music being best is the reason* the concert was amazing, which is incorrect. "although she feels that": This phrase introduces a concession or a contrasting idea. It means "despite the fact that." Karen agrees the concert was amazing, although* she feels their earlier music is still the best. This fits. The two correct answers are: but in her opinion although she feels that